- work on plugin user-friendliness (short help & co)

Added an interface to use C integers instead of GMP integers in Smith Normal
Form methods (disabled now, should be set to get enabled when GMP library
is not present).

Rather than imitating MElements, each Cell maps to an existing or newly
created MElement and uses MElement's services to their extend.
This brings simplicity and memory efficiency.

Code cleanup, increased verbosity. 

default after homology computatation for better generator chain visualization. 

Changed homology generator chain mesh element orientations (determined by vertex order) so that
post-processing data isn't needed to save the generators, instead all the
elements have coefficient 1 now in the chain. Due to this also reverted
changes made to GModel writeMSH method in my earlier commit (they became obsolete).

Added option to save MSH without renumbering the mesh elements (required to
save post-processing $ElementData right!).

Clean-up & small syntax change for homology commands.

Added homology computation tutorial as tutorial/t10.geo.

- display all control points in GUI for gmsh curves
- uniformize Fl_Value::step() values in GUI

- print num vert+elm after each generation
- reork tutorial a bit

pp

Layers{N,1} can now be written simply as Layers{N}

physical entities can now be defined using a string instead of a number (and
there is a new field $PhysicalNames in the .msh format to keep track of these
strings)

- re-add names in visibility broswer (will change in the future)
- shorten tutorials

better tutorial on transfinite grids

remove more attractor crap

don't use the "manual" numbering the extrude demo: it creates bad habits
for new users

fixed ugly extrusion syntax (finally!)

extrusions are now specified in the same way as all other transformations
(the old syntax is still available, but is deprecated)

refactored Opengl_Window.handle()

should behave exactly like the old version, but now it is at least (somewhat)
readable and ready to be generalized for more complicated projection matrices