Changing gradient function's output in differential_geometry

[JulienLefevreMars]

The gradient function that returns the gradient vector field of a scalar field returns a dictionnary of vectors. This data structure is probably not very efficient and could be replaced by a numpy array of size (nbpoints,3).

See also the VectorField class that is supposed to come soon ;-)

[JulienLefevreMars]

Two steps for that:

• the lazy one: convert the final dictionary in a numpy array
• the more efficient one: rewrite the code to get rid of the for loop and use ony numpy.array operations In both cases we need a unit test