Deduplicate common parts in LatexManager.{__init__,_setup_latex_process}

[anntzer]

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).

[tacaswell]

When do we make the LatexManager This is moving looking up the pgf command from when we create it to when it is called. Is that going to change any behavior?

[anntzer]

We always access LatexManager via _get_cached_or_new(), which drops the previous LatexManager instance and constructs a new one whenever the texcommand (or the preamble) changes.