Do not overwrite models with same number in PDB file

[Daniel Dudola]

Example PDB file:

1. MODEL 1
2. MODEL 2
3. MODEL 1
4. MODEL 2

MODELs 1 and 2 get overwritten on the file level with 3 and 4, respectively. This probably causes further issues.

[Daniel Dudola]

It is probably better to let the user fix it herself :)