Construct BioUnits from assymetric unit and MMCifInfoBioUnit object

- Required data is extracted from MMCifInfoBioUnit object and fed into a
  custom BUInfo object
- BUInfo object can be serialized into bytestring
- Chain names in biounit have following naming scheme:
  <idx>.<asu_chain_name>. idx -> n-th copy of ASU chain with n
  starting from 1.
- Properties are copied from whatever is stored in assymetric unit
- Chain properties copied: ChainType (CHAINTYPE_POLY etc.)
- Residue properties copied: OLC, sec struct, ChemClass (PEPTIDE_LINKING etc.),
  ChemType(AMINOACIDS etc.), IsProtein flag, IsLigand flag
- Atom properties copied: name, element, occupancy, bfactor, ishetatm