Added Geo/Homology.h and Geo/Homology.cpp as mid-interface for homology
computation (the role of utils/misc/celldriver.cpp so far). From there on
it's easy to either make plugin interface or parser interface, or both.

Made Homology Computation plugin usable.

The IO & graphics for post-processing views of the results of the homology
computation are crude at the moment: it saves the results to a separate
.msh file and reads them back by merging that to the current Gmsh session
(it uses File->Merge method).