Rename SparsePoint to something else

[Luthaf]

We are using the historical name of sparse points when working with GAP potentials. However, this name can be confusing since the "sparse" kernel is not a sparse matrix in the usual sense, only a low-rank one.

I think we should change the name to something clearer. One alternative is Pseudo-points, which have been used a lot in the group already, but @max-veit does not like it (I'll leave him explain why below!).

Another alternative is basis points, which is very clear in the GPR framework, although I thing basis is already used a lot in this code.

I personally like support point quite a lot: it tells what it is without being too ambiguous. The only source of confusion I could see is with support vector machines.

Do you have better alternatives, or arguments for keeping the "sparse point" naming?

[felixmusil]

You can add 'inducing' points to the list of potential rename. That being said I would prefer to avoid inventing a new name since the littérature already accepts a few names for the same object (support and basis have not been used as far as I know). Moreover I would not rename this object either since this is one of the accepted terminology so it's clear enough what this is.

[max-veit]

The terminology of 'active points' is also gaining currency FWIW. But personally I don't see a problem with calling them sparse points. The linear-algebra sense of 'sparsity' is a very narrow, technical definition that doesn't have the same meaning in other fields; it makes perfect sense to call the GPR active/inducing points 'sparse' since they're usually chosen (sparsely!) from the training data.