create_bayes_lm_data: Create input data frame for Bayesian regression
Description
This function "widens" focal-competitor data frames for use inside of
package modeling functions, where each comp_sp inside of the comp
list-column receives its own column with its associated total basal area.
This function is used internally by comp_bayes_lm() and
predict.comp_bayes_lm() exported as a convenience for
applications extending this package's functionality.