Gemini URL List download

This implements a URL List download at the base of the project files. It checks various http header information (if it's a fits, a gzip, the uncompressed md5) with the actual downloaded file and the database.

There are the beginnings of a restructure in this PR - it will not change any of the GMOS MOS parts, but make sure that GMOS imaging can reuse the MOS parts that are similar.

@mhvk your thoughts?