isochrones: parallax support; command-line scripts

Support has now been added to include parallax (in miliarcsec) to the observable properties of a StarModel. There are now also two command-line scripts, starfit, which allows executing an MCMC fit of a StarModel from the command line (e.g. starfit mystar where mystar is a folder containing a star.ini file), and batch_starfit, which executes many starfits at once on a cluster (currently assuming slurm is the batch scheduler, but presumably could be adjusted to any other)