Add param so we can run a prev simulation starting from a specific ti…

[lungd]

…mestep.

[pgleeson]

@lungd Are you working mainly with the development branch? Note the changes in this PR: https://github.com/openworm/sibernetic/pull/133 are the only ones I'm confident (at the moment) work correctly with c302...

[lungd]

@pgleeson I'd say, I mainly work with the ow-0.1 branch from my repo where I also merged your PR.

I just realized that this PR is not working as intended. When I start an existing simulation with the new param, the displayed time(step) is correct but the rendered worm is still in a position as if I would have started the simulation from the beginning (or without the new param)

[lungd]

Can be merged now