Additional features

[Pweidemueller]

• include option to return names of empty column
• devide each step into find and apply, e.g find_empty_columns and then remove_empty_columns etc.
• should we offer extra functionality to add an intercept to the design matrix before making it full rank? Should we have an extra check for a superfluous intercept in the matrix?

df <- data.frame(col = sample(letters[1:3], 10, replace = 20))
mat <- cbind(1, model.matrix(~ col - 1, data = df))
make_full_rank_matrix(mat)

• interface for formulas @const-ae