Documenting learning equations

[lecfab]

Purpose of this PR

Add to in-code documentation a summary of the file calculate_learning_cost.pdf written by @cchrisgong and I when trying to understand learning equations and floor costs.

Remaining issue: the file equations.gms does not seem to be part of automatic documentation generator.

Type of change

• Refactoring

Checklist:

• My code follows the coding etiquette
• I performed a self-review of my own code
• I checked that the in-code documentation is up-to-date
• All automated model tests pass (FAIL 0 in the output of make test)