Implement plane-to-plane transformations for tangent plane projections

[astrofrog]

It could be useful to implement an alternative pixel_to_pixel which implements the fast reprojection algorithm mentioned here:

http://montage.ipac.caltech.edu/docs/algorithms.html

specifically for WCSes with the same coordinate frame and in a specific subset of projections, it should be possible to do much faster computations of pixel-to-pixel transformations.

[astrofrog]

In fact, perhaps this new pixel_to_pixel could live in astropy.wcs, but I'll keep the issue here for now as we could develop it here first to try it out.

[astrofrog]

To start off I think we could implement it just for TAN which I think should be the easiest