set ligand_chains to an empty list

[mgiulini]

Desired feature/enhancement

Currently the default value of ligand_chains is a list containing the value "B". This value is in fact ignored in caprieval when multimeric docking is performed, so as to have RMSD values that correspond to the whole complex. It would be better to change this parameter to an empty list, so that the logic would simply be

IF empty list THEN ligand_chains equal to all the existing ligand chains.

In case the parameter is used, we stick to it (for example ligand_chains = ["B"])

Motivation

This would enhance HADDOCK's flexibility.