FData class

[pablomm]

In the branch feature/fdata I created the class FData to unify the API of FDataBasis and FDataGrid. It is just a suggestion of a possible API. I would like to discuss which methods are more appropriated to be in the common class and the interface of the methods.

In particular the api of evaluate and the parameter keepdims.

https://github.com/GAA-UAM/fda/blob/859bb4055bd9a325fb1321ddb6b4d761013489ad/fda/fdata.py#L47-L50

[vnmabus]

First, remove the constructor, as FData is an abstract class.
Second, I think the properties should be abstract.

I think I agree with the other proposed changes.

[pablomm]

Closed in #26.