Automate thinning and burn-in cuts for emcee

Currently, emcee and ptemcee just take nburn and nsteps as arguments. We could simplify this by just taking nsteps, then automatically trimming and thinning by the autocorrelation length (as calculated from the emcee.acor module.

• Also make a method for calculating autocorrelation time from the result object itself

mentioned in commit 515d3360

mentioned in merge request !112 (merged)

closed via merge request !112 (merged)

closed via commit 515d3360

mentioned in commit 17dc18b7

unassigned @gregory.ashton