Data model for emline files

[weaverba137]

Please review the data model for the emline files in both healpix and tiles directories:

The structural description of these files should be complete, but descriptions of the HDUs, header keywords and columns need work. If necessary it is possible for files to refer to other files, or for HDUs to refer to other HDUs, to reduce duplication of effort.

[weaverba137]

Closed by #128.