Provide default downsampling logic in Coreg pre-processing for performance

[rhugonnet]

Many users run into memory issues because their reference DEM is of higher resolution than the to-be-aligned DEM, and Coreg resamples everything on the reference.
We could add a "default" routine that always re-samples to the lowest resolution datasets, which can be overridden by the user with an argument during fit() that could something like resample_on : Literal["ref", "tba"]