Improve ProblemBuilder API

This simplifies the implementation and usage of problem builders. In particular, methods SetVarBounds, SetObj, SetCon, SetLogicalCon, GetLinearObjBuilder & GetLinearConBuilder have been replaced with AddVar, AddObj and AddCon.

Moved .nl-specific functionality to ProblemBuilderToNLAdapter.
Handle common expressions in ProblemBuilderToNLAdapter, fix solver tests
Fix comments
Update ilogcp-test