Improve Coreg docstrings

[rhugonnet]

Maybe to-do after things have changed in the next weeks.

          Maybe a minor comment. Should we expand the docstrings at the beginning of each coreg submodule to better explain what it contains?

E.g in base.py: """Base coregistration classes to define generic methods and pre/post-processing of input data.""" -> add a short description of the base Coreg class and attributes, the Pipeline and the Blockwise? Or is it too redundant with the doc and or the specific docstrings?

Originally posted by @adehecq in #424 (comment)