Added a module docstring.
Removed the empty __init__() method which was killing the base class method.
Removed the unused 'index' arg.
Fixed the call to the 3D isosurface mapping code.
The open_write_file force arg has been converted to a bool.
Fixed the calls to relax_io.open_write_file().
Renamed all refs to 'file' to 'file_prefix'.
Changed the 'res_num' arg to 'spin_id'.