contribution: Image warping routine?

Hi @jakirkham,

I have some potential contributions to make here as I am currently using dask to write a parallel image registration library.

Right now I have an implentation of image warping that you might be interested in. It allows warping with a vector field of pixel displacements (like skimage.transform.warp) by creating a large linear operator matrix using scipy sparse and then multiplying with the flattened image. It can only go as fast as scipy's sparse matmul implementation, but I couldn't find a faster way because of the horrible nature of the memory access pattern for arbitrary image warping.

If you are interested please let me know and I'll shape it into a PR.