Manual termination

[lindonroberts]

Implement a custom exception which allows the manual termination of the solver within an objective evaluation.

i.e. If evaluating objfun(x) raises a "TerminationException", then just terminate the solver immediately (returning the best point so far, etc.). Then users can put a "rase TerminationException" into their objective function code to quit for any reason.

[jeancroy]

If the objective function decides that, then maybe the objective function could have some report of the progress ?