Add option to save motion correction object

[EricThomson]

Would be nice to add ability to save motion correction object the way Caiman already saves the CNMF object. Motion correction can take a really long time, and it would be helpful to save/load these results in addition to the motion corrected file (which is not always wanted -- it is already optional to save the mc file).

Most obvious use case is if you want to apply motion correction from a structural channel to a functional channel at a later time, but in general it would be good to have option to save discovered parameters of the major estimator objects after the fit operation is run.

Relevant discussion: #1238