Add functionality to pass a `pesummary` results file to `ligo-skymap-from-samples`

An example pesummary results file can be seen here: https://ldas-jobs.ligo.caltech.edu/~cbc/pe/O3/S190408an/samples/posterior_samples.json. There is a function in pesummary which will convert this to a lalinference hdf5 file. This can be seen here https://git.ligo.org/lscsoft/pesummary/blob/master/pesummary/gw/file/existing.py#L134. Alternatively, it is easy to recover the samples for ra, dec and distance by simply reading the json file either using pesummary or just using the json package. I am happy to be assigned to this issue.