Return a list of tuples:
r_sq_interp_idx_coef_list = [ (r_idx_low, r_idx_high, coef_low, coef_high,), ... ] where: r_sq_interp_idx_coef_list[r_sq] = (r_idx_low, r_idx_high, coef_low, coef_high,) r_sq ranges from 0 to int(r_list[-1]**2 + 1.5)
r_sq_interp_idx_coef_list = [ (r_idx_low, r_idx_high, coef_low, coef_high,), ... ]
r_sq_interp_idx_coef_list[r_sq] = (r_idx_low, r_idx_high, coef_low, coef_high,)