Make it easier to programmatically modify hyperparameters

[grahamgower]

#223 and #308 would be dramatically simplified by passing hyperparameters to a function that returns an instance of a DinfModel. When I tried something like this, the multiprocessing code couldn't find the generator function. But I really should try to figure out some solution.

[grahamgower]

It seems that the multiprocess implementation is rather good, in that it works with functions nested inside functions, class methods, etc. What doesn't work is doing this stuff inside a Jupyter notebook (which would be ideal for docs purposes). I don't think there's anything more to be done here, unfortunately.