Add set_description to `pbar._NoOpPBar`

[andyfaff]

I'm interested in using the set_description method of a tqdm progressbar. If I do that it may make sense to add an empty set_description method to the emcee.pbar._NoOpPBar, so there's no issue if I try to call the method on a dummy placeholder object.

Would such an addition be welcomed, or would you prefer if I handle the absence of the method in user space?

[dfm]

I'd be happy to include that - thanks!