Cite stats authors for code in predict.factanal

[drsimonj]

Predicting factor scores is a challenging issue. Existing approach makes use of code from stats::factanal that is partially copied and pasted. See: https://github.com/drsimonj/twidlr/blob/master/R/twidlr-stats.R#L334-L340

The stats authors must be cited appropriately.