combining 'Distinct' with 'By' and column select shows some shortcomings
needed to add __slots__ to Distinct
expr: move count, distinct, nunique from column to table
python: column of column
expr: add TableExpr.iscolumn property
expr: TableSymbol.iscolumn keyword argument
compute: use iscolumn to resolve Distinct(column) issue
Merge pull request #339 from mrocklin/iscolumn

Add `iscolumn` property to TableExpr