- 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

what is done for ellipse arcs in the built-in kernel. Fixes #578.

Interface OCC sewing algorithm in the creation of surface loops, with extra arg in API or "Using Sewing" keywords in .geo files (fixes #583)

new mesh::clear()

Rationale: setNodes/setElements were intrinsically a bad design choice,
as these functions had to delete nodes and/or elements before storing the
new ones. Using these functions in the wrong way could lead to an invalid
mesh, with elements pointing to nodes that were deleted (in an adjacent
entity).

The only viable workflow is to delete the mesh (with the new clear()
function), before adding nodes/elements.

This reverts commit 07cc55d9

This reverts commit 211fa521, reversing
changes made to 8ba56d73.

modify getJacobians, getBasisFunctions & co to take integrationPoints directly as argument (instead of integrationType)