WIP: improvements to SDA to have non-annual time steps

[bailsofhay]

This PR add's new changes to the SDA that allow the user to have non-annual time steps and non-symmetric observed data.

Description

When finished, this PR will allow users to run the SDA at daily/sub-annual time steps. Different variables can have differing time steps from each other, as well as NA data.

Motivation and Context

The SDA can only run data at the annual time scale right now, and all variables have to run on the same dates, which doesn't always make sense.

Review Time Estimate

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

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X] 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.
• [ ] 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.

[github-actions[bot]]

This PR is stale because it has been open 365 days with no activity.

[mdietze]

Similarly, @helge22a you should look at this too to see if there's anything that needs to be pulled in to get the daily forecast SDA working

[mdietze]

@helge22a not immediately pressing, but before you wrap up it would be good to figure out what in this PR still needs to be integrated into the mainline SDA and what's already there. We need to either fix this PR or close it if everything is covers has been implemented elsewhere.

[helge22a]

It appears the only piece of code here that we do not currently have is the use of the read_output function in read.restart. Given Hamze's comments and all the improvements made to read.restart in recent PR, I do not think there is any new code here that needs to be included in our current workflow. I think it is safe to close this PR.