Make functions in `solvers/utils.py` private.

[bthirion]

          I agree, most functions in `solvers/utils.py` are not meant to be public (because they are called through `fugw.mappings` instances, which themselves call `fugw.solvers` instances).

I added a docstring, and suggest we hide all non-public functions in a follow-up PR.

Originally posted by @alexisthual in #29 (comment)