Fix install of python package

- the AND NOT in the cmake guard does not work anymore
- As we cannot negate UG_ENABLE_PARALLEL, we define a new cmake variable
and use that one as cmake guard

Predicate in GridGlue's Codim1Extractor now is just a std::function, so
we shouldn't derive from it and just define operator().

We only use this in nonconforming settings, where we can handle this.

New master