coded a .msh file reader

So far this targets the Gmsh
files generated by the neper program.
We assume linear meshes and that
mesh entities are specified if and only
if they are classified on an equal
order geometric entity.
coded a .tess file reader

This is a format emitted by the neper
program which is easier to parse than
Gmsh's .geo files, which are a whole language.
We only care about the subset that defines
the tesselation polyhedra.

It also made sense to separate gmi's
readers into another source file.
registering tess reader for meshmodel
fix bug in tess reader
debugged .msh reader, it works!