The computed `albumtotal` field should have numeric type

### Problem

When using `albumtotal` in a query, numeric ranges are not supported:

```sh
$ beet list -a -f '$album: $albumtotal' albumtotal:26 Petra
The Praise Collection: 26

$ beet list -a -f '$album: $albumtotal' albumtotal:23..28 Petra
(no output)
```

I would expect these two commands to give the same result.

### Setup

beets version 1.4.5
Python version 3.6.2
plugins: discogs, duplicates, embedart, fetchart, lastgenre, lyrics, missing, web

My configuration (output of `beet config`) is:

```yaml
lyrics:
bing_lang_from: []
auto: yes
bing_client_secret: REDACTED
bing_lang_to:
google_API_key: REDACTED
google_engine_ID: REDACTED
genius_api_key: REDACTED
fallback:
force: no
sources: [google, lyricwiki, musixmatch]
directory: ~/Music/beet
library: ~/Music/beet.db

plugins:
- discogs
- duplicates
- embedart
- fetchart
- lastgenre
- lyrics
- missing
- web
web:
host: 127.0.0.1
port: 8337
cors: ''
reverse_proxy: no
include_paths: no
duplicates:
album: no
checksum: ''
copy: ''
count: no
delete: no
format: ''
full: no
keys: []
merge: no
move: ''
path: no
tiebreak: {}
strict: no
tag: ''
discogs:
apikey: REDACTED
apisecret: REDACTED
tokenfile: discogs_token.json
source_weight: 0.5
user_token: REDACTED
missing:
count: no
total: no
album: no
fetchart:
auto: yes
minwidth: 0
maxwidth: 0
enforce_ratio: no
cautious: no
cover_names:
- cover
- front
- art
- album
- folder
sources:
- filesystem
- coverart
- itunes
- amazon
- albumart
google_key: REDACTED
google_engine: 001442825323518660753:hrh5ch1gjzm
fanarttv_key: REDACTED
store_source: no
embedart:
maxwidth: 0
auto: yes
compare_threshold: 0
ifempty: no
remove_art_file: no
lastgenre:
whitelist: yes
min_weight: 10
count: 1
fallback:
canonical: no
source: album
force: yes
auto: yes
separator: ', '
prefer_specific: no
```

Start by reproducing the two `beet list` commands from the report and inspect the query handling for the computed `albumtotal` field. Confirm that exact-value and numeric-range queries return the same album, then add or update coverage for both forms if the relevant tests are found.

Written by the indexing model from the issue text.