Add healpix map interface

[kadrlica]

I think the current plan for DES is to use the Oracle database as the primary repository for healpix maps. In order to facilitate user access to these maps, I think we want to provide a set of upload_map and download_map functions.

upload_map: Allow a user to upload a healpix map to the DESDM database. Would automatically take care of explicit pixel specification scheme and converting to nested formatting (the "standard" for DESDM healpix maps). Would be accessible from the command line and would take the map filename and desired tablename as inputs. Everything else could be dealt with under the hood with healpy. It would also be nice for this function to automatically set some of the table description information based on information collected from the uploaded file.

download_map: Wrapper to download a map into a fits (or fits.gz) file. Would automatically take care of converting to implicit pixel specification, nside specification, and ring/nest conversion. Would be accessible from the command line and would take as input the tablename and output filename, with options for ring/nest formatting. Most things would be dealt with by healpy under the hood.