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. 

Please note that objects from non native adapters will not necessarily of type Face but they will fullfill the Face interface. So raising in this case is wrong. The `from_face` classmethod used lated will check the interface indirectly, so there is not need to raise.