structure/SequenceMixin: add resi_begin and resi_end parameters to PDB loader Optional arguments resi_begin and resi_end where added to methods set_pdb_chain() and auto_pdb() of SequenceMixin. These arguments can be used to restrict range of pdb residues, that can be connected with monomers from sequence. These arguments are tuples like (hetero_flag, sequence_id, insertion_code). See biopdb documantation for more information. closes #61