Change command line options

At the moment arguments are passed to PESummary in the following format:

$ summarypages --labels one two --samples example.hdf5 example2.hdf5 \
               --psd H1:IFO0_psd.dat L1:IFO1_psd.dat H1:IFO0_psd.dat L1:IFO2_psd.dat

However, I am thinking that maybe there is a better way of passing the arguments to PESummary from the command line. Perhaps something like:

$ summarypages --labels one two --one_samples example.hdf5 --two_samples example2.hdf5 \
               --one_psd H1:IFO0_psd.dat L1:IFO1_psd.dat --two_psd L1:IFO1_psd.dat V1:IFO2_psd.dat

The advantage of this new way of passing the command line arguments is that it is clearer which arguments are for which label. Also it allows you to pass different sets of psds for different labels which is currently not possible. @vivien @cjhaster @virginia.demilio @joseph.mills thoughts?