Alt Methods for predict.factanal

[drsimonj]

predict.factanal currently implements the "regression" method directly from stats::factanal (when scores = "regression"`). See https://github.com/drsimonj/twidlr/blob/master/R/twidlr-stats.R#L334-L340

stats::factanal also provides the means for computing factor scores using the Bartlett method, which can be implemented.

If done, be sure to add method as an additional argument to predict.factanal, and be sure to cite the authors (see #17)