Resolves #266 and improve saved log likelihoods and priors

• For the emcee, ptemcee and cpnest samplers. Adds the log_likelihood and log_prior per sample to the posterior (for CPNest this was already there, but was named differently).
• For all other samplers, the log_prior is calculated after the run based on the samples.
• Modified the prior.PriorDict.ln_prob method to include an axis argument

Closes #266 (closed)