Removing alpha-spheres from AlphaSpheres class

[dprada]

We need three new methods in AlphaSpheres.

• remove_alpha_spheres: to remove a set of alpha-spheres given a list of alpha-spheres indices.
• remove_small_alpha_spheres: to remove those alpha-spheres with a radius smaller than a given length.
• remove_big_alpha_spheres: to remove those alpha-spheres with a radius larger than a given length.

Please @casuarioq, complete the docstrings of these new methods using the Numpy style.

¿Why do we need these methods? You can look for the answer in the description of the algorithms used by Fpocket or AlphaSpace.