Improve tests in test_affine once core class restructured with BiasCorr parameters

[rhugonnet]

The logic in test_biascorr can be copied to test all types of inputs (raster-raster, point-raster, point-point), but for efficiency we should only do this once the function class parameters are fixed (fit optimizer, binning, etc) to avoid rewriting the tests twice. See #435.

[rhugonnet]

And in more details, some other to-do in those classes once restructured:

• Homogenize tests in test_affine,
• Check that xdem.coreg.ICP fails very underscriptively when not enough points are available #422 and xdem.coreg.ICP.fit_pts fails for unknown reason with a regular dataset #423 are resolved (probably fixed by Fuse Coreg point functions and add consistent Raster-Point logic #480).