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. 

I would avoid checking the type (see comment above). I would suggest to add a new contructor (i.e. classmethod) in Point named `from_point`. That method could look like

```python
@classmethod
def from_point(cls, point):
    return cls(id=point.id, coordinates=point.coordinates, data=point.data, past_data=point.past_data)
```

This way if the `point` variable does not support the point interface we will get an attribute error. This pattern checks the api compliance indirectly and there is no need for a deepcopy later.