Read assemblies from PDB/MMTF files and symmetry mates from PDB files

[padix-key]

This PR adds get_assembly() and list_assemblies() to structure.io.pdb and structure.io.mmtf analogous to the functions in structure.io.pdbx.
In addtion, it adds get_symmetry_mates() to structure.io.pdb.
As foundation for the changes in structure.io.pdb PDBFile now also offers the get_remark() method to parse REMARK
records.

Furthermore, a MemoryError issue in structure.repeat_box(), that appeared on structures with associated BondList, was fixed.