start setting up some post processing

See merge request Monash/peyote!25

# Conflicts:
#   peyote/sampler.py

Marginalisation

See merge request Monash/peyote!27

# Conflicts:
#   peyote/prior.py
#   peyote/sampler.py
#   tutorials/BasicTutorial.py

Add ptemcee sampler

See merge request Monash/peyote!24

Allows a full set of data to be downloaded once, and then subseuqnt
analysis to simply cut from that file. Needs to be better intergrated
into the whole system.