Clarify/document various IDs and Indices and Numbers used internally

[jphickey]

Checklist (Please check before submitting)

• I reviewed the Contributing Guide.
• I reviewed the README file to see if the feature is in the major future work.
• I performed a cursory search to see if the feature request is relevant, not redundant, nor in conflict with other tickets.

Is your feature request related to a problem? Please describe.
The SC source code uses lots of different values to identify RTSs, ATSs, and Commands within them. Some are 0-based (usually for indexing into internal arrays) and some are 1-based (usually for telemetry).

But usage isn't entirely consistent. The type used to store these varies - sometimes uint8, but sometimes uint16 or even a signed value, with conversions/assignments between them, scattered all over the source.

There are conversion macros (e.g. SC_ATS_ID_TO_INDEX) but these do not seem to be applied totally consistently. Furthermore, there is some overlap between these - SC_RTS_INDEX_TO_NUM and SC_RTS_INDEX_TO_ID appear to do exactly the same thing. NUM and ID both appear to refer to a 1-based identifier and there is no documented distinction here why these two exist.

Describe the solution you'd like
First and foremost - Document what is meant by an "ID" and an "INDEX" and a "NUM" and where each one should be used. Scrub through the code and make sure that conversion macros are consistently used, and the right variant is used in the right context. Make sure a consistent data type is used to store the identifier.

Furthermore - to improve code robustness, at least Internally within SC - use a unique (type-safe) data type to refer to each identifier. This will restrict implicit conversions or comparisons between them, allowing any mismatches to be caught at compile time. This ensures that any conversion is intentional, and must use the correct conversion macro to perform the conversion.

Requester Info
Joseph Hickey, Vantage Systems, Inc.