return updated pars array from wrapPars rather than update in-place

wrapPars is used to update the pars array and the stored chain, but
there is no setter for sampler.flatchain attribute. Instead, have
wrapPars return an updated array, then assign that the sampler._chain
after reshaping.

This should fix an issue with stored chains having unwrapped PA
values, which seemed platform-dependent. (#16)