Change DSSP format

[touwwouter]

The transition from the old PDB format to mmCIF/PDBx poses some issues:

• Multi-character chain identifiers in mmCIF files do not fit the single column in the current DSSP format (see #74)
• In the current mmCIF/PDBx dictionary the are _atom_site.label_<...> and _atom_site.auth_<...> data items to identify _atom_sites. To provide backward and forward compatibility it would make sense to include the mandatory items and the currently used items in a new DSSP format (the current DSSP format also lacks model numbers, for example).

Perhaps a command line option format can be added with two allowed values, new and old (default).

[cbaakman]

Version 3 of xssp produces new formats now.