update todo

done.txt

@@ -7,3 +7,4 @@ x 2015-12-26 clean up tox files, make sure all tests run             @tests
 x 2016-01-03 check divfree function
 x 2016-01-03 compute kMeta(t) as well
 x 2016-01-03 split library into core and extra                                       @optimization +v1.0
+x 2016-01-07 FFTW interpolator doesn't need its own field                            @optimization +v1.0 +particle_api

todo.txt

-(B) FFTW interpolator doesn't need its own field                            @optimization +v1.0 +particle_api
 (B) add stat choice parameter to add_particles                              @design +v1.0 +particle_api
 (B) simplify tracer/field addition mechanism                                @design +v1.0 +particle_api
 (B) compute z polynomials only when needed                                  @optimization +v1.0