additional metadata to be included in the result .h5 file

Before we move to sampling codes that provide everything directly in their outputs, it'd be nice to have the possibility to give to pesummary (on the command line for instance) additional files and meta-data, so that its output is as complete as we want. For instance, we could then give pesummary, in addition to the samples to plot:

• PSD files
• Calibration envelopes
• config file
• downsampled h(t) itself