Get how often the Jacobian is rebuilt. See
SNESGetLagPreconditioner() to determine when the preconditioner is rebuilt
snes - the
lag - -1 indicates NEVER rebuild, 1 means rebuild every time the Jacobian is computed within a single nonlinear solve, 2 means every second time the Jacobian is built etc.
The default is 1
The jacobian is ALWAYS built in the first iteration of a nonlinear solve unless lag is -1 or
SNESSetLagJacobianPersists() was called.