Add loglikelihood evaluations to pymultinest

Once the run is complete, read in the log likelihood from the phys_live.points file and store it in the usual way.

For future reference

1. This document shows how pymultinest provides an interface to analysing the data. But, it doesn't provide an API to the max likelihood
2. This page provides the information about what how the files are written.

Note, this also changes how the samples are read in to avoid any mismatch in ordering definitions.