Benchmarking Tools

This isn't really an issue so much as a place to discuss comparison between tools.

I have two reprojection routines in [FITS_tools](https://github.com/keflavich/FITS_tools) now, both under the label 'hcongrid'. The AST version is capable of transforming between projections (gal<->cel, etc.), while the scipy `map_coordinates` version requires that the input and output be in the same system. At the moment, the initial routine Tom has written up in #25 is loosely equivalent to hcongrid.

https://github.com/keflavich/FITS_tools/blob/master/FITS_tools/hcongrid.py

Start by reading FITS_tools/hcongrid.py and the discussion in issue #25. Compare the AST-based and scipy map_coordinates routines described here, including their projection limitations. This issue has no defined implementation target or completion criteria, so agreement on the benchmark scope and expected outcome would be needed before work can be considered done.

Written by the indexing model from the issue text.