Add a helper to generate mathtext error strings.

[anntzer]

This also ensures consistency in the names in the error strings; see
e.g. changes in test_mathtext (the contents of \overline are indeed a
"body", not a "value").

Renaming "simple_group" to "opt_group" is not only to make definitions
of over/underset fit in a single line, but also because the name seems
semantically more meaningful too. (req_group, OTOH, is just for
conciseness.)

PR Summary

PR Checklist

Tests and Styling

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• New features are documented, with examples if plot related.
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).
• Documentation is sphinx and numpydoc compliant (the docs should build without error).

[timhoffm]

Is there an important reason for the abbreviation? I'd have gone with required_group and optional_group.

[anntzer]

This makes more of the definitions below fit in a single line (e.g. making the definitions of frac/dfrac/binom and overset/underset more clearly parallel each other). I can undo that if you feel strongly about it, though.

[timhoffm]

I prefer the full name, because that's more readable. I think wrapping is not too bad for the mentioned cases:

       p.binom <<= cmd(
           r"\binom", p.reqired_group("num") + p.reqired_group("den"))

[anntzer]

done