-nter and -cter flags do not work for multi-chain molecules

[pckroon]

For proteins that consist of multiple chains which are connected through e.g. a disulfide bond the -nter and -cter flags don't work. They'll only apply to the terminal residues with the lowest/highest resid. This means that only 2 out of 4 (in case of two chains) termini will get patched. The selection of n/cter should be changed to compare the resid of the terminal residue to the resid of the neighbouring residue, rather than min/max.