Build a sorted list of unique basis functions based on columns, where each basis function is a list
make_basis_list(X_sub, cols)
| X_sub | A subset of the columns of X, the original design matrix. |
|---|---|
| cols | An index of the columns that were reduced to by sub-setting. |
Note that sorting of columns is performed such that the basis order equals cols.length() and each basis function is a list(cols, cutoffs).