Don't transform underlying datasets

Description

Refactored xarray_fmrc to no longer depend on some geometry of underlying datasets. Instead it should be able to get more information from the structure of the datatree (forecast_reference_time). If more tweaking is needed, things like the time coord, time format, group_prefix, and forecast_coords can be overridden on the accessor, and it will attempt to those configs from top level datatree attributes.

Additionally it should be possible to set model_run_path and forecast_reference_time_from_path to more fully tweak how xarray_fmrc interprets the structure of the datatree.

Related Issue

Closes #10 Closes #8

Type of Change

[ ] 📚 Examples / docs / tutorials / dependencies update
[ ] 🔧 Bug fix (non-breaking change which fixes an issue)
[ ] 🥂 Improvement (non-breaking change which improves an existing feature)
[ ] 🚀 New feature (non-breaking change which adds functionality)
[x] 💥 Breaking change (fix or feature that would cause existing functionality to change)
[ ] 🔐 Security fix

Checklist

[x] I've read the CODE_OF_CONDUCT.md document.
[x] I've read the CONTRIBUTING.md guide.
[ ] I've updated the code style using make codestyle.
[ ] I've written tests for all new methods and classes that I created.
[ ] I've written the docstring in Google format for all the methods and classes that I used.

abkfenris / xarray_fmrc