InsertionCode handling

[av-luebben]

As SHELXL does not recognize insertion codes, pdb2ins will renumber residues which are inserted.
Each insertion code will get an offset starting with 1000, added to their residue number.
e.g. residue PRO A: 234 A will get A = 1000 added to give PRO A:1234.
residue ALA A: 234 B will get B = 2000 added to give ALA A:2234.