Making residual/jacobian zero-outing configurable

The motivation behind this patch is the following.  In one of our setups, the jacobian is constant over the whole simulation (it is a mass matrix in an explicit scheme). In order to save the computation time of re-evaluating the jacobian, we want to compute it only once and reuse it, thus we need a way how to tell libmesh not to zero it out. This simple patch does it.

In order to maintain symmetry, this patch does the above for the residual as well, even though I am not quite sure, if it is even helpful/usable in any scenario.