Improve depiction of Latex in GAMS-created docs

[glatterf42]

This PR puts all strings with more than two characters in \text{}, which should help the depiction of long formulae in particular. Note that some single characters have seen the same fate, especially in macro_core.gms, because it would have appeared inconsistent to have some parameters in a table be in \text{} and not others if they represent the same thing thematically.

How to review

• Read the diff and note that the CI checks all pass.
• Build the documentation and look at the 'Mathematical Specification' pages.

PR checklist

• Continuous integration checks all ✅
  Except for macos-latest-py3.7 per Track/work around actions/setup-python#682 ixmp#484.
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update release notes.

[codecov]

Codecov Report

Merging #721 (587cc00) into main (3cd004a) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #721   +/-   ##
=====================================
  Coverage   94.4%   94.4%           
=====================================
  Files         43      43           
  Lines       3448    3448           
=====================================
  Hits        3257    3257           
  Misses       191     191           

[khaeru]

Thanks for thoroughly going through this, @glatterf42.

Approved, with one minor request:

• Update release notes.

I think it's worth advertising this, as IMHO it's a significant improvement in readability. It could be like:

- Improve readability of LaTeX equations in docs (:pull:`721`).

Some other notes for posterity:

• We had a discussion in Slack about the merits of using \text{…} explicitly (e.g. \text{beyond_horizon_factor} versus expanding this code in the Sphinx config

  message_ix/doc/conf.py

  Lines 141 to 157 in 3cd004a

  	# TODO read at least some of these from message_ix.models
  	# TODO complete list
  	# TODO also add these to a LaTeX preamble
  	text_macros = """ACT
  	STORAGE
  	STORAGE_CHARGE
  	duration_time_rel
  	input
  	map_time_commodity_storage
  	storage_initial
  	storage_self_discharge"""
  	mathjax3_config = dict(
  	tex=dict(
  	macros={k.replace("_", ""): r"\text{" + k + "}" for k in text_macros.split()},
  	),
  	)

  which would define a TeX commands (e.g. \beyondhorizonfactor). We decided for the former for now a variety of reasons.
• Building PDF version of the docs from LaTeX fails locally #722 is related but (IMHO properly) kept separate and not scheduled for now.