Move from emcee to ptemcee

Previously, we used the emcee.PTSampler, but this has been removed from
the master branch of emcee (see discussion here
https://github.com/dfm/emcee/issues/236) and a fork, ptemcee has been
developed. Testing shows this is equivalent (after some changes in the
interface) and perhaps better as it contains the adaptive temperature
ladders developed by Will Vousden (the maintainer of ptemcee)