Fixes triggered regridding on identical grids issue

[ks905383]

• Moves process_weights, which homogenizes weights and the input ds, from after subsetting to a bounding box to before that process, which prevents regridding to be triggered on otherwise identical weights and ds grids, if a bounding box is used (which can lead to an error if xesmf is not installed)
• Adds a warning if inputted weights don't span the entirety of the ds (this happens irrespective of the extent of the geodataframe inputted, however, which could lead to spurious warnings if the weight file spans the geodataframe but the ds covers a larger area)
• Adds tests to make sure the above warning is correctly triggered and weights and ds are jointly subset to bbox correctly.

Closes #86