For most uses of the TS solvers the user need not explicitly call
TSSetStepNumber(), as the step counter is appropriately updated in
TSSolve()/TSStep()/TSRollBack(). Power users may call this routine to
reinitialize timestepping by setting the step counter to zero (and time
to the initial time) to solve a similar problem with different initial
conditions or parameters. Other possible use case is to continue
timestepping from a previously interrupted run in such a way that TS
monitors will be called with a initial nonzero step counter.