Add downscale_function and covariates scripts

[JoshuaPloshay]

Description

Motivation and Context

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.

[mdietze]

@JoshuaPloshay this PR is failing because of a conflict between the updated Roxygen and the not-yet-updated man page. Could you pull the changes I've made into your local branch, then rerun make document, then commit the updated NA_downscale.Rd?

[mdietze]

@JoshuaPloshay I believe you have code that loads up existing PEcAn output, calls your downscale function, and does some analyses and visualizations of the outputs (including quantile checks). It would be good to add such code to this PR (e.g. in the /inst folder) so that others don't need to recreate this to be able to pick up on where your analyses left off.