Improve handling of extra arguments (e.g. from ini files)

[brendan-m-murphy]

Scikit learn has a system where keywords are prepended with the name of the function that they should be passed to.

We could probably implement this using the sections in the ini files. E.g. infer_pymc__nuts_sampler = "numpyro" would pass nuts_sampler = "numpyro" to infer_pymc, and the prefix could be added from the section via

[infer_pymc]
nuts_sampler = True

The existing system works, but we need to manually pass parameters to the right locations after they are given to fixedbasisMCMC.