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. 

update:
```python
if face_ids is None:
    for face in self.faces
        yield Face.from_face(face)
else:
    for face_id in face_ids:
         yield Face.from_fame(self.faces[face_id])
```