FIXED BUGS:
- directed was not set when reading from cm file
- pdbChainCode not set when reading from db given pdbCode
- filling pdbresser2resser and resser2pdbresser hashmaps also in reading from pdb file
- using "A" as chainCode when reading from pdb file
- some error handling for file formats: new exception classes GraphFileFormatError and PdbfileFormatError