move physical group definitions to new discrete entity created by compound meshing constraint, except for surfaces when we do the "magic" reclassification on the original surfaces

add ERROR_QUIET option to execute_process(git log ...) to ignore errors caused by faulty git installations

* new separate option to govern curvature adapted meshes (Mesh.MinimumElementsPerTwoPi instead of MininumCirclePoints)
* -clcurv command line option now takes Mesh.MinimumElementsPerTwoPi as argument

- new fltk/isAvailable to detect if GUI has been closed

Quitting the GUI now simply destroys the GUI, without exiting Gmsh. This allows to create/destroy the FLTK GUI at will in an API script.

  Less_Edge -> MEdgeLessThan, Equal_Edge -> MEdgeEqual, Hash_Edge -> MEdgeHash
  Less_Face -> MFaceLessThan, Equal_Face -> MFaceEqual, Hash_Face -> MFaceHash
  MVertexLessThan -> MVertexPtrLessThan, MVertexEqual -> MVertexPtrEqual, MVertexLessThanLexicographic -> MVertexPtrLessThanLexicographic
  Less_ElementPtr -> MElementPtrLessThan
  Less_Cell -> CellPtrLessThan
  GEntityLessThan -> GEntityPtrLessThan
  compareMLinePtr -> MLinePtrLessThan
  equalMLinePtr -> MLinePtrEqual

Physical is a reserved keyword of the .geo language - use PhysicalGroup instead (cf. #614 and a0d5ff7b)

this allows to easily separate first order from high-order nodes later on (using
mesh/getElementsByType), and is less intrusive than modifying mesh/getNodes & co to
return the information directly