Add ability to specify additional latex packages

[A-CGray]

Purpose

Adds an optional keyword argument to the XDSM initializer that allows the user to specify additional LaTeX packages they want pyXDSM to use when creating the pdf and tex files of their diagram.

Expected time until merged

Not super urgent, but also not a big PR, max 2 weeks?

Type of change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (non-backwards-compatible fix or feature)
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Documentation update
• Maintenance update
• Other (please describe)

Testing

Checklist

• I have run flake8 and black to make sure the code adheres to PEP-8 and is consistently formatted
• I have run unit and regression tests which pass locally with my changes
• I have added new tests that prove my fix is effective or that my feature works
• I have added necessary documentation