Allow bias-correction methods to be called from residuals

[rhugonnet]

Brought up by @jbelart: Bias-correction methods are not specific to two DEMs, they work on residuals of two datasets, here elevation differences.

Right now, it is not possible to call a BiasCorr method from a dDEM, which could be preferable for users. We could add this possibility.
In time, we might think about moving this kind of bias-correction method to GeoUtils, as they are generalizable to other data?

In the meantime, a trick to use xDEM's current BiasCorr on elevation differences (or any residuals): provide residuals for ref_elev, and a matrix of zeros for tba_dem!