msaexp: NIRSpec analyis tools

Docs now at https://msaexp.readthedocs.io/ What's Changed Fix bugs with slit metadata in MSA metafile by @gbrammer in https://github.com/gbrammer/msaexp/pull/20 Add docs files by @gbrammer in https://github.com/gbrammer/msaexp/pull/21 Full Changelog: https://github.com/gbrammer/msaexp/compare/0.5...0.5.1Please cite this software using these metadata