Ligo skymap optimise

The purpose of this MR is to add two additional command line arguments. The first --np_for_skymap will allow the user to choose how many cores they would like to use to generate the ligo.skymap. The second --nsamples_for_skymap will allow the user to specify how many samples they would like to use when generating the ligo.skymap. This will involve randomly sampling the posterior distributions for ra, dec and distance. @gregory.ashton