Add an option to output the residue indexes in each leaflet.

Hi,

I add an option (called ``print-residue`` or ``-i`` to print the residue id (or indexes) into the standard error at the end of all outputs.
It could be useful to quickly identified which lipids are part of each leaflet.

To avoid modify to heavily the code, I create a new function ``split_resid`` based on ``split``. This function is called in ``split_leaflets`` which now take another argument and return in all cases 2 dictionnaries : one with the standard atoms group and the other one with the resid groups. The resid groups dictionnary is empty when the option is not set.