fix dangerous clean

make sure we define  YY_NO_UNISTD_H on wind32

A minimal Gmsh "null" model can now be embedded with no external dependencies

API cleanup: new discreteVertex, discreteEdge, ... entities (replacing
the old hacks in gmshVertex, gmshEdge, ...), the future basis of the
indepedent "null" model (cf. HAVE_NO_GEO define)

API cleanup: FVertex, FEdge, etc. -> fourierVertex, fourierEdge, etc.

API cleanup: removed unused functions