Interpolate by projection onto an ellipsoid

[Banana1530]

Hmmm. Let me think more about this - I'm pretty busy for the next few days, so we can leave this as an issue and come back to it in a different PR.

Originally posted by @michaelweylandt in https://github.com/michaelweylandt/MoMA/pull/42

[michaelweylandt]

Elsewhere you mentioned scaling instead of proper projection: that's a clever idea and has the advantage of not changing sparsity patterns. No firm thoughts one way or the other yet - just recording for posterity.