Documentation for RefNLPEP2D forward operator and variable transforms

[steve-marshall-piq]

Description

The provides documentation for the RefNLPEP2D forward operator and its associated variable transforms.

Issue(s) addressed

There are no issues related to the PR

Dependencies

This is related to a PR in the ufo repo that added the code for the RefNLPEP2D forward operator, and another PR in the ufo-data repository that added the unit test data for this software.

List the other PRs that this PR is dependent on:

• [ https://github.com/Addition of the RefNLPEP2D GNSSRO forward operator ufo#77]
• [ https://github.com/Added unit test data for RefNLPEP2D forward operator and associated variable transforms JCSDA-internal/ufo-data#535]

Impact

No downstreams effects expected on other repositories.

Checklist

• [X ] I have performed a self-review of my own code
• [X ] I have made corresponding changes to the documentation
• [ X] I have run the unit tests before creating the PR