Support WHERE statements.

[ikeough]

WHERE statements are currently parsed, but not code is generated. These should generate data validation code which is fired when an entity's state changes.

[ikeough]

In discussion with @jmirtsch, it was decided that these validations would be good to have as static functions on the 'Model' class (or whatever class manages the collection of IFC object instances). These validations could then be run as part of the transaction which commits the instances.