give fit_MA_mv() a back transformation argument

[egouldo]

• fit_MA_mv() application for yi Eucs to be shifted into ManyEcoEvo (requires back-transformation of data)

Originally posted by @egouldo in #29

The plots for yi Euc required back-transformation to response scale, move this functionality into software.

Note: ⚠ leave this code in manuscript at least for now, since our focus is getting off to reviewers.

[egouldo]

Has been addressed in #121 #118 - back-transformation happens prior to model fitting, and user-supplied columns allow different outcome variables, e.g. mean_log