Fix writing of SEQRES for chains with n*13 residues (bug #366)
Fix writing of SEQRES for chains with n*13 residues (bug #366)

The chain serial number was not correctly incremented for
chains with a multiple of 13 residues. This lead to corrupt
SEQRES records when writing PDB files.

Reported by Peter Ebert.

tkemmer/ball,tkemmer/ball,tkemmer/ball,tkemmer/ball,tkemmer/ball,tkemmer/ball