Adding SDA restart capability

[helge22a]

Description

Condensing sda.enfk_MultiSite function with new functions metSplit, sda_matchparam, and build_X, added more comments to hopefully make this easier to work with. Now can run SDA at any time step (not only annual). Added new nimble model for running one site at a time through assimilation.

Motivation and Context

All changes necessary for scaling up SDA to continental scale/adding more data streams for NASA CMS project with @DongchenZ @serbinsh @mdietze @Qianyuxuan

Review Time Estimate

• [ ] Immediately
• [ ] Within one week
• [ ] When possible

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] My change requires a change to the documentation.
• [ ] My name is in the list of CITATION.cff
• [ ] I have updated the CHANGELOG.md.
• [ ] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[serbinsh]

Wow @helge22a this is a LOT of work and a really nice PR! lots of great contributions here! I did my best to review but not sure i can dig deeper without a better understanding of running it all