Step Time Schema Improvements

[cartermak]

Capturing ideas from a couple different conversations for consideration here.

1. Enforce in the schema that absolute, epoch-relative, and command-relative times all have the tag field populated (Dupe: Enforce time tag for types ABSOLUTE, COMMAND_RELATIVE, and EPOCH_RELATIVE #24)
2. Enforce a format on the tag fields (e.g., YYYY-DOYTHH:MM:SS[.sss] for absolute, [DOYT]H:MM:SS[.sss] for command-relative, and [+|-][DT]HH:MM:SS[.sss] for epoch-relative).