interface: nearest neighbour atom pairlist between chains

Information of interface residues in complexes can be obtained with switch
--interfaceOut, yielding the output file 'interface.out',
For each atom of the input structure, the nearest neighbour
is computed based on the neighbour list and printed if the two contact
atoms are in different chains. Note that this pairlist is not symmetric:
atom distance A-B might be shortest for A, but distance B-C might be shortest
for B. Also, the list might change with protein motion. Therefore it depends
on the interpretation to be derived from the interface whether it is suitable
to use the atom pairs, the residue pairs and/or their distances.

Excerpt of output for '1f3r.pdb':
1 N TRP A 67 3570 O ASN B 231 3.0156
7 CD1 TRP A 67 1069 OG SER B 61 3.3179
9 NE1 TRP A 67 1069 OG SER B 61 2.8603
30 CB ASN A 68 992 CE1 TYR B 56 3.6573
32 OD1 ASN A 68 1029 N TYR B 59 3.1650
33 ND2 ASN A 68 1112 NZ LYS B 64 3.3937
43 O PRO A 69 990 CD1 TYR B 56 3.1214
....

The first 5 columns are descriptors of atom 'i', the second 5 columns are descriptors of atom 'j',
the last column is the distance between 'i' and 'j'.