Refactor ReducedDatums to a multi-model based infrastructure

[jchate6]

We want to refactor Reduced Datums to have more structure.
We will have 4 types of Reduced Datums in tom Base:

• General (Anything not otherwise covered)
• Photometry
• Spectroscopy
• Astrometry

Users are expected to create their own models for more complex forms of calculated data.

See:

• Make Data validation and control class for Reduced Datums #609
• Creating ReducedDatums too slow in the age of LSST #1480
• Model Field Breakdown Sheet