fix column keys so that query grouping works

[snopoke]

Previously the following query would result in 2 SQL queries instead of 1 because the column key hash did not work:

vc = QueryContext("user_table", filters=filters, group_by=["user"])
vc.append_column(SumColumn("indicator_a", filters=[GT('date', 'enddate')]))
vc.append_column(SumColumn("indicator_b", filters=[GT('date', 'enddate')]))

[snopoke]

This is the main change along with the implementation of correct hashing and comparison for the filter classes

[snopoke]

just for debugging

[orangejenny]

For the __hash__ implementations, is there a reason that sometimes they use type(self) and sometimes they use the class name?

[snopoke]

I used type(self) for base classes (though I see I copied that to the INFilter class as well.