Define standard units and some standard keynames for use in JSON

ADR-35 decided on guidelines for JSON key names. This ADR goes two steps further:

1. Agreeing on the standards for units used for physical quantities;
2. Beginning a dictionary of JSON key names that are common across telescopes and system components

Why Now? Agreeing these soon will prevent future re-work

Scope: All software systems

Quality: Usability, Debuggability, Consistency