Add `iscolumn` property to TableExpr

This fixes an issue brought up by @talumbau

```Python
>>> # Handles obvious cases
>>> accounts['name'].iscolumn
True
>>> accounts[['name', 'balance']].iscolumn
False

>>> # less obvious cases handled by standard method recursion up the tree
>>> accounts['name'].distinct()iscolumn
True
>>> accounts[['name', 'balance']].distinct().iscolumn
False
```