Make new animation including old png's and new frames from restart

[rjleveque]

If make plots has been used to make some png files and then more output frames are added by doing a restart, it would be nice to make a new animation that includes all frames without remaking the png files that already exist (for cases when each plot takes a long time to produce).

A bit complicated to make this work smoothly both in serial and parallel plotting mode.

Requested by @mjberger