pair_dict should be a dict mapping tuple (HET code, residue ID) -> (HET code, residue ID) e.g. {('MG ', 'A 204 ') : ('MG ', 'C 221 '), ...}.
           HET codes and residue IDs should respectively correspond to columns 17:20 and 21:27 of the PDB file.

def from_tuples_dict(pair_dict):
        '''pair_dict should be a dict mapping tuple (HET code, residue ID) -> (HET code, residue ID) e.g. {('MG ', 'A 204 ') : ('MG ', 'C 221 '), ...}.
           HET codes and residue IDs should respectively correspond to columns 17:20 and 21:27 of the PDB file.
        '''
        lm = LigandMap()
        for k, v in pair_dict.iteritems():
            lm.add(k[0], k[1], v[0], v[1])
        return lm