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

[HubLot]

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.

[coveralls]

Coverage remained the same when pulling 550a208 on HubLot:master into b9db4d5 on jbarnoud:master.