ENH: add random ellipsoid phantom

[kohr-h]

This might be useful.

[adler-j]

Overall very good, but since this is actually somewhat speed critical (people would use it in e.g. ML) I think we need to be more careful. Specifically we should try to do away with all the "while" loops, especially globally. If we cannot do that, at least move them locally.

[kohr-h]

I admit this is the lazy implementation. I'll make one that checks overlap with the coordinates only.

[kohr-h]

I've done some work on this a while ago, but it turned out that it's nontrivial to efficiently determine whether an ellipsoid intersects another one, apart from trivial situations. I've got one pending idea that's not implemented yet, which is to transform the coordinates such that the "query ellipsoid" becomes a ball -- this reduces the problem to determining the distance of an ellipsoid to the origin.

Other suggestions are welcome.