save progress with mxTryHard

mxTryHard (and variants) don't save their progress (no checkpoint=T) https://openmx.ssri.psu.edu/node/4642

Perhaps this could be done by expanding the loading and saving of models to also include past fitting attempts. This way mxTryHard can pick up where left off when interrupted, and I can also page through the solutions of all the tries that mxTryHard made.