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. 

"descriving" -> describing   I see also: realtive/interator/insted/implentation.   maybe open in a text editor that automatically highlights misspelled words and review the docstrings for any others.