Add descriptive statistics to data dictionary

Woohoo! The semi-auto, resource-level data dictionary is here!
https://github.com/ckan/ckan/pull/3414

We should consider expanding the data dictionary with more descriptive statistics by using a tool like https://csvkit.readthedocs.io.

Perhaps, as a datapusher "post-processor", we can call csvstats asynchronously to compile descriptive statistics.

For example, this sample file from data.boston.gov ([rptcityscoresummary.csv.zip](https://github.com/ckan/ideas-and-roadmap/files/877419/rptcityscoresummary.csv.zip)
taken Mar 28), we got this csvstat report ([rptcityscoresummary-descstats.txt](https://github.com/ckan/ideas-and-roadmap/files/877421/rptcityscoresummary-descstats.txt) ). It took a minute on a MacBook 2016 to scan the file and create the report.

```
1. "CTY_SCR_NAME"

Type of data: Text
Contains null values: False
Unique values: 22
Longest value: 34 characters
Most common values: CITY SERVICES SATISFACTION SURVEYS (313x)
GRAFFITI ON-TIME % (313x)
MISSED TRASH ON-TIME % (313x)
PARKS MAINTENANCE ON-TIME % (313x)
POTHOLE ON-TIME % (313x)

2. "CTY_SCR_NBR_DY_01"

Type of data: Number
Contains null values: True (excluded from calculations)
Unique values: 1673
Smallest value: 0
Largest value: 11,854
Sum: 2,662,821.802
Mean: 445.586
Median: 1
StDev: 1,913.897
Most common values: 1 (1100x)
None (620x)
0 (523x)
2 (113x)
0.667 (79x)

3. "CTY_SCR_NBR_DY_02"

Type of data: Number
Contains null values: True (excluded from calculations)
Unique values: 492
Smallest value: 1
Largest value: 3,295
Sum: 463,537
Mean: 122.499
Median: 16
StDev: 301.434
Most common values: None (2812x)
4 (157x)
6 (152x)
5 (145x)
8 (143x)

4. "CTY_SCR_NBR_WK_01"

Type of data: Number
Contains null values: True (excluded from calculations)
Unique values: 3141
Smallest value: 0
Largest value: 10,138.5
Sum: 2,959,883.164
Mean: 465.025
Median: 0.94
StDev: 1,912.761
Most common values: 1 (255x)
None (231x)
0 (193x)
0.143 (91x)
0.857 (84x)

5. "CTY_SCR_NBR_WK_02"

Type of data: Number
Contains null values: True (excluded from calculations)
Unique values: 944
Smallest value: 1
Largest value: 14,056
Sum: 2,540,635
Mean: 622.4
Median: 73
StDev: 1,466.989
Most common values: None (2514x)
39 (46x)
43 (45x)
51 (45x)
29 (42x)

6. "CTY_SCR_NBR_MO_01"

Type of data: Number
Contains null values: True (excluded from calculations)
Unique values: 4781
Smallest value: 0
Largest value: 9,624.613
Sum: 2,976,713.567
Mean: 462.151
Median: 0.934
StDev: 1,906.679
Most common values: None (155x)
0.097 (63x)
1 (22x)
0.129 (21x)
0.065 (19x)

7. "CTY_SCR_NBR_MO_02"

Type of data: Number
Contains null values: True (excluded from calculations)
Unique values: 1566
Smallest value: 11
Largest value: 33,897
Sum: 10,682,411
Mean: 2,610.56
Median: 290
StDev: 5,961.668
Most common values: None (2504x)
141 (21x)
169 (21x)
180 (20x)
253 (20x)

8. "CTY_SCR_NBR_QT_01"

Type of data: Number
Contains null values: True (excluded from calculations)
Unique values: 5218
Smallest value: 0.044
Largest value: 9,408.122
Sum: 2,982,826.846
Mean: 459.462
Median: 0.926
StDev: 1,898.813
Most common values: None (104x)
0.12 (44x)
6.533 (32x)
6.267 (29x)
0.109 (26x)

9. "CTY_SCR_NBR_QT_02"

Type of data: Number
Contains null values: True (excluded from calculations)
Unique values: 2084
Smallest value: 52
Largest value: 78,741
Sum: 31,401,398
Mean: 7,673.851
Median: 851
StDev: 17,285.547
Most common values: None (2504x)
469 (13x)
725 (13x)
663 (13x)
858 (12x)

10. "CTY_SCR_TGT_01"

Type of data: Number
Contains null values: True (excluded from calculations)
Unique values: 8
Smallest value: 0.75
Largest value: 95
Sum: 29,202.15
Mean: 6.631
Median: 0.8
StDev: 21.411
Most common values: 0.8 (2508x)
None (2192x)
0.95 (382x)
4 (340x)
6 (313x)

11. "CTY_SCR_AVG_01"

Type of data: Number
Contains null values: True (excluded from calculations)
Unique values: 1507
Smallest value: 0.05
Largest value: 8,826.548
Sum: 2,575,791.761
Mean: 1,175.624
Median: 56.053
StDev: 2,656.83
Most common values: None (4405x)
0.113 (29x)
0.097 (26x)
0.15 (25x)
0.129 (24x)

12. "CTY_SCR_AVG_02"

Type of data: Number
Contains null values: True (excluded from calculations)
Unique values: 1785
Smallest value: 0.087
Largest value: 8,633.411
Sum: 2,542,310.677
Mean: 1,160.343
Median: 55.594
StDev: 2,618.471
Most common values: None (4405x)
0.141 (17x)
0.136 (17x)
0.143 (14x)
0.132 (13x)

13. "CTY_SCR_DEV_01"

Type of data: Number
Contains null values: True (excluded from calculations)
Unique values: 1860
Smallest value: 0.22
Largest value: 1,967.8
Sum: 485,499.32
Mean: 221.588
Median: 9.834
StDev: 523.541
Most common values: None (4405x)
0.319 (22x)
0.303 (14x)
0.349 (12x)
0.298 (12x)

14. "CTY_SCR_DEV_02"

Type of data: Number
Contains null values: True (excluded from calculations)
Unique values: 1983
Smallest value: 0.301
Largest value: 1,788.384
Sum: 493,031.198
Mean: 225.026
Median: 10.173
StDev: 527.044
Most common values: None (4405x)
0.309 (11x)
0.342 (8x)
0.341 (8x)
0.347 (7x)

15. "ETL_LOAD_DATE"

Type of data: DateTime
Contains null values: False
Unique values: 313
Smallest value: 2016-01-15 00:00:00
Largest value: 2017-03-28 00:00:00
Most common values: 2017-03-23 00:00:00 (22x)
2017-03-24 00:00:00 (22x)
2017-03-10 00:00:00 (22x)
2017-03-13 00:00:00 (22x)
2017-03-15 00:00:00 (22x)

16. "ETL_LOAD_IS_ACTIVE_FLAG"

Type of data: Boolean
Contains null values: False
Unique values: 2
Most common values: False (6574x)
True (22x)

17. "CTY_SCR_OPEN_DATA_SOURCE"

Type of data: Text
Contains null values: True (excluded from calculations)
Unique values: 5
Longest value: 100 characters
Most common values: https://data.cityofboston.gov/City-Services/311-Service-Requests/awu8-dc52 (2136x)
https://data.cityofboston.gov/Public-Safety/Crime-Incident-Reports/7cdf-6fgx (2136x)
None (1790x)
https://data.cityofboston.gov/Permitting/Approved-Building-Permits/msk6-43c6 (267x)
https://data.cityofboston.gov/City-Services/Boston-Public-Library-Daily-Active-User-Counts/mzws-sfys (267x)

18. "CTY_SCR_METRIC_TYPE"

Type of data: Text
Contains null values: True (excluded from calculations)
Unique values: 3
Longest value: 10 characters
Most common values: Target (3761x)
Historical (1869x)
None (966x)

19. "CTY_SCR_DAY"

Type of data: Number
Contains null values: True (excluded from calculations)
Unique values: 2818
Smallest value: 0
Largest value: 15.061
Sum: 6,335.908
Mean: 1.158
Median: 1.083
StDev: 0.489
Most common values: None (1125x)
1.25 (870x)
0.938 (77x)
1 (77x)
0.833 (63x)

20. "CTY_SCR_WEEK"

Type of data: Number
Contains null values: True (excluded from calculations)
Unique values: 4195
Smallest value: 0.25
Largest value: 61.434
Sum: 7,785.7
Mean: 1.261
Median: 1.082
StDev: 1.048
Most common values: None (424x)
1.25 (181x)
0.938 (32x)
0.833 (25x)
1 (25x)

21. "CTY_SCR_MONTH"

Type of data: Number
Contains null values: True (excluded from calculations)
Unique values: 5516
Smallest value: 0.362
Largest value: 20.055
Sum: 9,059.189
Mean: 1.408
Median: 1.081
StDev: 1.411
Most common values: None (161x)
0.968 (19x)
0.97 (18x)
0.93 (16x)
0.952 (15x)

22. "CTY_SCR_QUARTER"

Type of data: Number
Contains null values: True (excluded from calculations)
Unique values: 5846
Smallest value: 0.705
Largest value: 10.348
Sum: 8,639.596
Mean: 1.331
Median: 1.079
StDev: 0.85
Most common values: None (104x)
0.918 (32x)
0.957 (29x)
0.96 (21x)
0.976 (21x)

23. "CTY_SCR_DAY_NAME"

Type of data: Date
Contains null values: False
Unique values: 5
Smallest value: 0001-01-02
Largest value: 0001-01-08
Most common values: 0001-01-05 (1328x)
0001-01-04 (1328x)
0001-01-08 (1328x)
0001-01-03 (1306x)
0001-01-02 (1306x)

Row count: 6596
```

Review the existing resource-level data dictionary from pull request #3414, then examine Datapusher post-processors and the csvkit csvstats tool. Done means asynchronously compiling descriptive statistics and adding them to the data dictionary, with behavior and output defined for the example CSV.

Written by the indexing model from the issue text.