feat: ``prep7`` submodule

[clatapie]

Description

This PR follows the PyConverter-XML2Py integration plan to automate the PyMAPDL_commands documentation. The changes have been generated using pyconverter-xml2py and more specifically mapdl-cmd-conv.

This PR focus on the prep7 submodule.

Pinging @ansys/pymapdl-developers for visibility. Feel free to provide any feedback on the way the docstrings and the source code generation are handled.

Issue linked

This PR is meant to be merged within the feat/main_commands branch. The latter will gather all the submodule changes, one by one, prior to be merged to the main branch.

Checklist

• [x] I have visited and read Develop PyMAPDL.
• [x] I have tested my changes locally
• [x] I have added necessary documentation or updated existing documentation.
• [x] I have followed the PyMAPDL coding style guidelines.
• [x] I have added appropriate unit tests that also ensure the minimal coverage criteria.
• [x] I have reviewed my changes before submitting this pull request.
• [x] I have linked the issue or issues that are solved to the PR if any.
• [x] I have assigned this PR to myself.
• [x] I have marked this PR as draft if it is not ready to be reviewed yet.
• [x] I have made sure that the title of my PR follows Commit naming conventions (e.g. feat: adding new MAPDL command)