Fix some formatting in Registry for generating docs

[xylar]

To generate Latex documentation from the Registry, we need to escape Latex in the namelist and variable descriptions.

[xylar]

Here is the resulting user's guide:

mpas_ocean_users_guide.pdf

[xylar]

Here is the PR where I'm proposing to update the User's Guide using these changes:
MPAS-Dev/MPAS-Documents#77
As I mention there, the auto-generated documentation is constructed using the tool I updated here:
https://github.com/xylar/MPAS-Tools/blob/update-parse-registry/python_scripts/namelist_generation/parse_xml_registry.py

[xylar]

@mark-petersen, could you let me know if these changes look okay? If so, I'll move over to E3SM.

[mark-petersen]

Yes, looks good. Thanks!

[xylar]

Thanks @mark-petersen!

Replaced by E3SM-Project#6293