Reference schema for input catalogs

@danielsf and @KSK need to work together to define a standardized schema based on the DM schema, the current schema and the outcome of the census of observables needed by working groups to do validation of the catalogs. This should involve the CosmoSim group from DESC.

Requirements for the reference schema:

  1. Contains enough information to produce phosim/GALSIM input files
  2. Contains enough information to produce values for the project schema
  3. It must be extensible to include arbitrary physical properties: masses, halo props, etc.
  4. Should define a manditory minimal schema.