[FEATURE]: Pull in coordinates from separate file

Description

Some WRF data does not contain the lat/lon coordinates in the same file as the data variables. This was the case in this example. There is still the added step of needing to pull in this data before XWRF can effectively do its work (it still cleans up the time coordinates nicely). I have tested this in this notebook.

Implementation

Here is a sample function that adds cleaned up lat/lon coordinates when a file is missing them:

def assign_coords_from_file(file_path, geo_path):
    #open dataset
    ds = xr.open_dataset(file_path, engine="xwrf")
    
    #open coord file
    ds_geo = xr.open_dataset(geo_path, engine="xwrf")

    #add coords
    ds_coords = ds.assign_coords(lat=ds_geo.coords['XLAT'], 
    lon=ds_geo.coords['XLONG'])

    #rename coords to match CF conventions
    ds_rename = ds_coords.rename({'south_north':'y', 'west_east':'x'})

    #drop redundant coords
    ds_clean = ds_rename.drop(['XLAT', 'XLONG', 'CLAT'])

    return ds_clean

ds = assign_coords_from_file("data/T2_RR_F_2014_08.nc", "data/wrfinput_d02")

Tests

Perhaps we need to add a test that all variable dimensions are represented by a coordinate.

Questions

How common is it for WRF data to store the coordinate information in a separate geo-file?

Are there any patterns for where this geo-file is located that we can exploit to automatically find and pull in the correct geo-file without needing it provided?

Also I noticed that XWRF does not currently rename dimension names (south_north to y, e.g.). Is this something we want to do? It is done in the example.

Start by reviewing the example notebook and sandbox/sandbox.ipynb, then inspect the proposed assign_coords_from_file flow for opening the data and geo-files, assigning lat/lon, renaming dimensions, and dropping redundant coordinates. Define the intended geo-file discovery and dimension-renaming behavior, and add a test that every variable dimension has a coordinate. Done means separate-coordinate WRF data is handled consistently with agreed behavior.