PESummary formatting

Closes #4 (closed)

Add option to format PE so that each event has its own hdf5 file with a fake supername, and the file structure is:

MDC -----------------------------------> group
MDC/posterior_samples -----------------> group
MDC/posterior_samples/mass_1_source ---> dataset
MDC/posterior_samples/<parameter> -----> dataset
MDC/injected_values -------------------> group
MDC/injected_values/mass_1_source -----> dataset
MDC/injected_values/<parameter> -------> dataset