Cys bonds not correctly recognized in .gro files and .pdb files without chain identifier

[Djurredejong]

Cys bonds and links given with the options -cys chain/resn/resid,chain/resn/resid or -link chain/resn/resid/atom,chain/resn/resid/atom,bondlength,forceconstant are not correctly recognized for gro files (where no chain can be defined) and pdb files where the chain identifier field is empty.

This can be circomvented by explicitly defining an empty chain identifier on the commandline:
-cys " /CYS/15, /CYS/48".

However, it would be better to internally better handle empty chain identifiers: they should always be None (this is currently done correctly in def str2atom but incorrectly when reading in .gro/.pdb files.

[Djurredejong]

Fixed by commit 4ab1164.