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!

- better (?) check for MKL without Fortran on MSVC

Changed set(GMSH_API by list(append GMSH_API
Added DILevelset.h to GMSH_API if ENABLE_DINTEGRATION

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

CMakeLists.txt to set the oofelie flags

re-addiing Fl_Tree and MatghEx after server crash

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