Add bounds to Minuit optimizer

[pelssers]

Passes bounds for each parameter to the Minuit optimizer by adding them to the iminuit dictionary.

[JelleAalbers]

Thanks Bart! This will ensure bestfit_minuit works again, and probably make it work more stable. I've added one more argument that may help get more meaningful errors out of minuit (although we currently discard those errors, so there is not yet much point).