Update function `extract_bounding_spectral_region` so as to handle spatial regions as well

[ibusko]

Function extract_bounding_spectral_region is supposed to replace minimal_subcube in spectral-cube, but falls short of that. It handles only the spectral dimension. It needs to be extended to handle spatial regions as well.

Note that this only makes sense when masks are supported for spectral cubes. In the spectral domain SpectralRegion acts as as de facto mask, but in the spatial dimensions the only way to generate an "empty" region is via masking.