Add List Logical Type

- When you have NaturalLanguage columns, you might want to calculate the average/median/total word length.
- To calculate all of those you would need to get the word length (for each word in the row value).
- We can therefore create a new List Logical Type, that will be a list of integer values, to handle this situation.
```python
class List(LogicalType):
primary_dtype = 'object'
standard_tags = {}
```
```
nlp word_length
1 having 3 words [6, 1, 5]
5 having 2 [6, 1]
9 having 4 words . [6, 1, 5, 1]
```