Take cached kernel_mean_row_sum in Refine

[tp832944]

Refine.refine() currently calculates kernel_mean_row_sum with no option to use an already-calculated value. Add this as an optional argument to the refine(), taking a default value of None to indicate that it still needs to be calculated. Document that Refine.approximate_kernel_row_sum will be ignored if kernel_mean_row_sum is passed.

This is necessary for OOPing of kernel herding refine.