[release/v1] Rules for "flag", "index", and other similar control variables

[mkavulich]

We have vaguely discussed a plan for handling "flag" variables, which I've been treating as a category including all variables that are specific to model code or other logical controls rather than physical quantities. This can include a number of possible variable types, often (but not always) indicated by a prefix:

1. Boolean indicators for whether a scheme, section of code, or other option is being exercised, or condition is met. Prefixes include flag_for_, do_, is_
2. Integer index of a particular list, table, or other data structure controlling code execution, file read/write, or other.... Prefixes include index_of_, control_for_, identifier_for_

The plan is to separate out all of these "control variables" into a dedicated section, with dedicated rules setting out their naming conventions, since these variables often do not fit neatly into the schema we have developed for naming physical quantities.