Re-document the in-built math

[Bryn Pickering]

Problem description

Now that we have YAML constraint definitions (#411), we need to automatically generate latex math versions of the constraints in the documentation without access to the docstring math (which is how the current math formulation documentation is built).

This requires the following steps to be completed:

• Add a description key to all custom math components, which we parse in generating the docs
• Add functionality to generate LaTex math strings from the constraints on evaluation of the final parsed expression (i.e., sub_expressions should already be replaced in the string)
• Write a script to parse and evaluate all in-built constraints (and custom in-built math formulation files - #424) and add to the docs.

In addition, the construction of these LaTex math strings should be available via a public API that can be used to generate a math formulation file which includes a user's custom math so they can e.g. put it into a paper supplementary.

Calliope version

v0.7.0-dev