Build a sorted list of unique basis functions based on columns, where each
basis function is a list
     
    
    make_basis_list(X_sub, cols, order_map)
 
     
    
    Arguments
- 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.
 
- order_map
 
A vector with length the original unsubsetted matrix X which specifies the smoothness of the function in each covariate.
 
 
    
    Details
    Note that sorting of columns is performed such that the basis order
equals cols.length() and each basis function is a list(cols, cutoffs).