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. 

Currently, __init__ has the parameter 'uuid' which is used to set the 'id' attribute.  I think here the parameter `uuid` should be renamed to `id` to be consistent with Particles.  Or alternatively, we could change Particles.  

Also,  `None` should be a  default argument value for the id/uuid parameter.

I think the Parameters section should be moved to '__init__''s docstring.  

Correct me if I am wrong on these suggestions.  These comments also apply to Element, Edge, Face and Cell