Constraints should be able to be applied at aggregate levels

[danfrankj]

e.g.

provider.evaluate('user', measures='count', where='transaction/count > 100')

Currently, transaction/count is applied at the transaction level, leaving us with no results as transaction/count has value 1 at the transaction level.

Instead we need a mechanism for enforcing constraints at the unit level to answer questions like ... "how many users are there with more than 100 transactions"

[danfrankj]

Perhaps add a "having" clause with the same constraint data structures