rbf → distance_functions Relation

File in include/num_collect/rbf	Includes file in include/num_collect/rbf/distance_functions
gaussian_process_interpolator.h	euclidean_distance_function.h
impl / get_default_scalar_type.h	euclidean_distance_function.h
rbf_interpolator.h	euclidean_distance_function.h
rbf_polynomial_interpolator.h	euclidean_distance_function.h