make MElement and MVertex thread-safe by guarding _globalNum

add benchmark/boolean

refactor occMeshConstraints + add missing writeOCC functions

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!

(Not sure if this is right. But the new ordering seems to match Code_Aster and SYNOISE?)

re-committing work from last two days after server crash:

- refactored old Geo code (new/delete)
- moved DrawPlugin to Plugin
- move from matheval to mathex
- move from Flu_Tree_Browser to Fl_Tree
- fixed crash in Triangulate plugin

- moved GUI to FlGui + added experimental QGui to test Qt

- fixed most MSVC and gcc compiler warnings

- converted some remaining tabs to whitespace (that's why the patch is
  so large; please try to remember to indent with 2 spaces and not
  tabs when you commit stuff to CVS)

- merged modified patch from Konstantinos Poulios to allow saving
  physical/partition as element tags in mesh formats that don't
  support them natively