varRegressors: Adds actual numeric regressors for factor to dataframe as new variables
Description
Adds new variables/columns in dataframe to represent numeric regressors for a factor.
Factors are coded using their currently defined contrast codes.
This function is useful for control of a factor covariate when graphing
and ignoring this factor and/or other lower-level control variables.
For this purpose, POC coding will typically be set for factor prior to using lm.codeRegressor