Convert .dat files to .hdf5 files

lalinference_O2 produced all of its results as .dat rather than .hdf5, and so cannot currently be ingested by pesummary. There are two options: 1) add functionality to pesummary to read the .dat files or 2) convert the .dat files to .hdf5 files. I suggest we go with option 2, since in the future we expect to have .hdf5 files.

I have hacked together a small script to convert LALInference .dat files to .hdf5 files, which is attached. It would be good to get this into pesummary. convert_dat_to_hdf5.py @charlie.hoy

similar to #79 (closed)