fix cl comp pb for embedded verts

rewrote gmsh2DMeshGenerator to make it thread-safe

*** PLEASE TEST THAT THE 2D MESH GENERATORS BEHAVE EXACTLY AS BEFORE ***

fix warnings

remove last statics to make BDS thread-safe

make MElement and MVertex thread-safe by guarding _globalNum

make divide & conquer thread-safe by encapsulating everything into the DocRecord class

oops, should not have committed openmp tests with jf

add benchmark/boolean

refactor occMeshConstraints + add missing writeOCC functions

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.