Add options to customize print statements of iteration function

[rhugonnet]

The _iterate_method added in #530 allows to have consistent iterations and parameters across all coregistration methods, but currently has fixed verbose print statements.
We should allow to customize what is printed by passing new argument to this function, which would be strings describing the nature of the iterating_variable and tolerance_statistic used during the iteration. For instance: "Horizontal offsets" and "Magnitude of horizontal offset" for NuthKaab.

Originally discussed here: #530 (comment)