before unraveling np functions in get_fit
changed from 312 to 300
unraveled np.sums, and slices for a_1, b_1
unraveled np.sums, and slices for a_2, b_2
unraveled slices and sums for b_3, a_3
unraveled slices and sums for RSS[j, k]
included finding min RSS in loop