Bug fix for the SHEET PDB records created by the structure.write_pdb user function.
The current and previous atom parts of the record were not being correctly formatted. This was
simply using the %4s formatting string. However the PDB atom format is rather more complicated. To
handle this, the new _handle_atom_name() helper function has been added to the
lib.structure.pdb_write module. This is now used in the atom() and sheet() functions for
consistently formatting the atom name field.
