Feature mesh

Dear all,
Here is the implementation of the stand-alone mesh.
Files:
 - abstractmesh.py: abstract interface for the class
 - mesh.py: current implementation of the mesh and the different types of elements.
 - test_mesh.py: the set of unitary tests for the class.

Please be wellcome to review the code and point out any suggestion or improvement.
Br. 

I would omit the references to length/area/volume in the documentation (and re-add them later when they are implemented in another PR)