update todo

todo.txt

 (B) FFTW interpolator doesn't need its own field            @optimization +v1.0
+(B) clean up tox files, make sure all tests run             @tests        +v1.0
 (B) compute z polynomials only when needed                  @optimization +v1.0
 (B) use less memory                                         @optimization
+(B) split library into core and extra                       @optimization +v1.0
 (C) clean up machine_settings mess                          @design @documentation +v2.0
 (C) code overview                                           @documentation
 (C) move stat I/O to cpp lib                                @design @HDF5