2d meshing algo with -optimize does lloyd optim (tri or quad optim algo depending if recombine is selected)

* issue error for zero volume elements in FEA export
* reset element number between 2D/3D in FEA export

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rewrote Compound interface in terms of standard elementary entities

Warning: this is work in progress and *will* break some existing examples (negative numbers etc.)

Added possibility to save distance to all boundaries (gmsh box.geo -2 -distance writes an additional distance.pos file)
distance is computed simply with ann for the moment

new "Delete Options;" command to reset all options to their default values

- invert #define logic (HAVE_MESH, HAVE_POST, HAVE_SOLVER, ... instead of HAVE_NO_MESH,
  HAVE_NO_POST, HAVE_NO_SOLVER)

Clean-up & small syntax change for homology commands.

Added parser interface for homology computation (finally).
A tutorial will follow.

renamed most gmsh* classes

better pre-fill of solver fields in GUI

stuff, like plugin drawing

Encapsulate toolkit-dependent global drawing functions in new
drawContextGlobal class, and add a static drawContextGlobal to
drawContext, with a default version that does nothing.

This allows to have a completely virtualized drawing system: we can
now ship a library with a fully working graphic system, indepedent of
the widget toolkit. A client code just needs to subclass
drawContextGlobal to get working graphics (see
e.g. api_demos/mainAntTweakBar.cpp)