restructuring some clipping code

[bbell]

Some minor changes should be made to improve the structure of the clipping code.

1. perform mask generation after checkType() or evaluate() in the QtCurtain class, not before.

2. remove deprecated query formats such as subspace(c, …) and update system tests to use only the newer formats such as clip(c, subspace(…) ), and update the corresponding wiki pages.

3. rename some of the variable types in oql.yy for more clarity, and introduce more documentation in the clipping parsing.

4. remove an erroneous/deprecated constructor in the QtClipping class.