io_data.atom_files_directory -- search path for coordinate files

This is a Python list of directories in which to search for coordinate files. (By default, only the current directory is searched.) PDB-style subdirectories (the first two characters after the digit in the PDB code) are also searched for each directory e.g., 1abc is searched for in the 'ab' subdirectory, pdb4xyz.ent in the 'xy' subdirectory.