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. 

Sounds good.  I just opened an issue ( #20 )  to clarify this "id/uuid name".  So we can address that issue of having a consistent name for the id/uuid in particlecontainer/mesh in a later PR.