WIP: blml specification of gpi format (experimental)

[cmungall]

This is an experiment in providing a more computable schema and data dictionary for gpi (and later gpad) files. It does not replace the markdown docs, and is not intended for any practical use any time soon.

Some potential advantages downstream:

• more computable unambiguous representation of datamodel
• formal mappings to other data models and dictionaries (BGI, GFF3, BL, ...)
• automatic generation of HTML, diagrams
• automated generation of json-schema and shex for checking
• generation of various other downstream artefacts