Add GriddedInterpolation for non-uniform grids

[timholy]

I was looking into doing a benchmark against other packages, and the lack of this feature made the comparisons pretty unfair. So I decided to add it. This was one of those days where one marvels at the efficiency of Julia.

I haven't done extensive testing or benchmarking, hence the WIP.

[timholy]

OK, I think this is ready to go. Testing could probably be better, but this is a good start.

[tomasaschan]

This is a nice refactoring! 👍

[tomasaschan]

This is really nice!

Re: testing, what is the expected behavior when the knots are actually linspace(1, size(A,d), size(A,d))? Could we make testing better by comparing a regular b-spline interpolation bitp = interpolate(A, BSpline{Linear}, OnGrid) with a gridded gitp = interpolate(A, Gridded{Linear}) and make sure that their results are within some small epsilon of eachother?

Also, we should start some feature-matrix somewhere, that indicates what features (interpolation degrees, gradient evaluation etc) that are implemented in the different interpolation schemes.

[timholy]

Could we make testing better by comparing a regular b-spline interpolation

Good suggestion. This caught what could have been an embarrassing bug.