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. 

However, It think the real problem here is the fact that points is not a list of ids but a list of references. It might be solved with `self.points = [Point.from_point(point) for point in points]` to make sure that all is copied. 

Yet. I feel that it is a sub-optimal solution, I will open a new discussion issue for further iterations.