extractVolume could miss some full (sub)elements for non-simplices

Improved reduction algorithm when generators of highest dimension are
present.
Added Betti number computation using coreduction to the UI.
Added support to quadrangles (not yet to prisms, hexaherdons or pyramids
due to unresolved issues).
Bugfixes.

levelset from postview !

add matheval levelset

more work on polyhedra

GModel::readGEO must be a static member function, since the parser can
now create multiple models!

more work on levelsets & co

missing return!