Add Support for multiple version of CV CMIP6, CMIP6Plus, CMIP7

[rbeucher]

This pull request adds support for the CMIP6Plus controlled vocabularies in addition to the existing CMIP6 and CMIP7 vocabularies. Users can now select between CMIP6, CMIP6Plus, and CMIP7 vocabularies by specifying the cmip_version parameter, and the documentation and codebase have been updated accordingly. Additionally, the CMIP6 vocabulary files have been removed from the repository, likely in favor of using the new submodule.

The most important changes are:

Feature: CMIP6Plus Vocabulary Support

• Added support for the CMIP6Plus controlled vocabularies throughout the codebase. Users can now choose "CMIP6Plus" as a cmip_version when initializing ACCESS_ESM_CMORiser, and the appropriate vocabulary class (CMIP6PlusVocabulary) is instantiated. (src/access_moppy/driver.py) [1] [2] [3] [4]

Documentation Updates

• Updated the documentation and example notebooks to explain how to select between CMIP6, CMIP6Plus, and CMIP7 vocabularies using the cmip_version argument, including usage tips and example code. (docs/source/getting_started.rst, notebooks/Getting_started.ipynb) [1] [2]

Repository Structure

• Added new git submodules for the CMIP6_CVs and CMIP6Plus_CVs repositories, ensuring that the controlled vocabularies are managed as external dependencies rather than tracked directly in the repository. (.gitmodules)

Cleanup: Remove In-Repo Vocabulary Files

• Removed previously tracked CMIP6 vocabulary files from the repository, likely because they are now provided via the new submodules. [1] [2] [3] [4] [5]

Will fix #222

[codecov]

Codecov Report

❌ Patch coverage is 76.38889% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.28%. Comparing base (7549b46) to head (cf0d58c).

Files with missing lines	Patch %	Lines
src/access_moppy/vocabulary_processors.py	75.00%	17 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #223      +/-   ##
==========================================
+ Coverage   47.47%   48.28%   +0.81%     
==========================================
  Files          22       22              
  Lines        4183     4239      +56     
==========================================
+ Hits         1986     2047      +61     
+ Misses       2197     2192       -5     

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