This returns the column name of the leftmost column of factors containing a
nonzero value in a row corresponding to a categorical variable.
h_first_term_containing_categ(factors, categorical)A string: one of the column names of factors. If none of the
columns contain a categorical variable, NULL is returned.
(matrix)
the factors attribute of a terms.object,
which is a matrix of 0s, 1s, and 2s.
(character)
a vector of the categorical variables in
the model whose terms.object is factors.