Ability to Customize DataSource Group By Clause by Dimension

[totalhack]

Currently the datasource level queries just group by the dimensions numerically (i.e. group by 1,2,3 if there are 3 dimensions in use). There are cases where you might want to be able to customize the clause that gets used here, such as adjusting/coercing column collation in MySQL at query time. The flexibility should maybe be limited here though, as screwing up the group by logic when trying to do something more complex could lead to unexpected behavior/output that might be hard to diagnose at first.