add option to write last snapshot before reaching end of simulation

[crpiceda]

In release 3.0.0, output_ox_last is only written at the timestep N where pb%it == pb%itstop (N=-1). It'd be useful to set N = -1 by default, which signals that a snapshot should be written at pb%itstop, but to have the option to write the output every N steps for N > 0 (including one at pb%itstop). The latter situation is useful when the simulation is running (i) through the wrapper and then it's interrupted; (ii) through a jobscript in the cluster and the job reaches the time limit before reaching TMAX.