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)

Since each SFT is generated in a for loop, the process can be quite
slow. This allows the user with `pathos` multiprocessing installed to
leverage `-N` cores (set by command line arg) to speed the process up
Setting N=1 or if pathos is not installed, reverts to the usual
behaviour.

Requirement is matplotlib>=1.4 in order to use the style package. Make the
default context ggplot (available in 1.4) and add an error message to
help the user when they have selected one which does not exist.

Makes this an optional module, adds it to the requirements and README.md