Optimization fixes - Githubissues

I'm not sure what's causing the optimization issues on Apollo but this PR makes some changes to the way the FE communicates with the BE in optimization. In particular

The elapsed time should no longer get faster and faster with successive runs
Optimizations can be cancelled

There's a chance that these changes modify/remove the code causing the issue and that the issue will be resolved automatically. If not, I'm hoping it'll be a little easier to isolate what's going wrong with the changes made here.

@cliffckerr (mainly) - cancelling tasks is accomplished via stoppingfunc but it's necessary to add some calls to stoppingfunc at various points in the code to achieve termination in a timely fashion. ASD has support for gracefully terminating (returning whatever it got up to) but as the FE shouldn't display partial results and cancelling takes effect immediately, it's equally fine to cancel via

    if stoppingfunc and stoppingfunc():
        raise op.CancelException

where CancelException is a new error class that is specifically caught in run_task(). I've dropped in a few of those checks in various places during minoutcomes (e.g. if you cancel a task, then it shouldn't try and start ASD for all of the ~11 runs or so only to have them each terminate after one iteration). So it would be great at some point if you could put a few of these checks into minmoney as well

optimamodel / optima

Optimization fixes #1823