Used by the cg solver only.
The conjugate direction is updated using a certain coefficient to the previous
direction. This coeffiction can be computed in different ways. The default is
to use Fletcher-Reeves (FR), an alternative is Polak-Ribiere (PR).
The coefficient for Fletcher-Reeves consists of the current norm of the
steepest descent vector divided by that of the previous iteration.
For the Polak-Ribiere scheme, a product of the current with the previous
steepest descent vector is subtracted in the nominator.