Parameters Able to Be Saved Back to Emcee

I've modified the call to the user's model slightly so that it gets the parameters back from it. Usually, these will be the same as whatever is passed in, but if the prior is a uniform interval, sometimes the user must modify the input value if it is outside the allowed range. This change enables this change to be saved back to the chain. Note this is similar to how LaplacesDemon works (the R package).

Probably the drawback to this is that currently users MUST pass back the parameters. It could be made so that it is optional.