LocalDescriptors num_neighbors has an unclear name

[Bradley Dice]

The property num_neighbors in LocalDescriptors isn't a very clear description - the value actually represents the "Last number of bond spherical harmonics computed" (as described in the C++ file), which is equal to the number of bonds in the computed neighbor list.

I suggest a rename to num_sphs or num_bonds or similar. To me, num_neighbors sounds more like the k in k-nearest-neighbors, which won't scale with the number of particles present.

    @property
    def num_neighbors(self):
        return self.thisptr.getNSphs()

    def getNSphs(self):
        warnings.warn("The getNSphs function is deprecated in favor "
                      "of the num_neighbors class attribute and will be "
                      "removed in a future version of freud.",
                      FreudDeprecationWarning)
        return self.num_neighbors

Thoughts, @vramasub @mspells?

[Bradley Dice]

...maybe this is fine. I read the Python docs again and it gives an explanation of the value, but I'm still not very fond of the property name. Current description:

 num_neighbors (unsigned int):
            The number of neighbors used by the last call to compute. Bounded
            from above by the number of reference points multiplied by the
            lower of the num_neighbors arguments passed to the last compute
            call or the constructor.

[Vyas Ramasubramani]

IIRC I rewrote both the cpp and python docstrings for this attribute to clarify precisely this point, but I didn't actually change the name. We could change the name too if we wanted to. We'd have to be careful not to clash with the existing naming of things in the Local Descriptors documentation is all.

[Matthew Spellings]

That quantity definitely shouldn't be called num_neighbors. It is the number of bonds, but I neglected to update the docstring in f9ea60ded when I updated the result of LocalDescriptors from being a Np*Nneigh*Nsphs array to a Nbonds*Nsphs array, causing the property to be added in 367edfdcf. There could conceivably be a value in returning the last num_neighbors value given in LocalDescriptors.compute, but I think it certainly shouldn't be spitting out the number of bonds when it's called what it currently is.

[Bradley Dice]

API change, punting to 2.0.