update todo

done.txt

@@ -10,3 +10,4 @@ x 2016-01-03 split library into core and extra
 x 2016-01-07 FFTW interpolator doesn't need its own field                            @optimization +v1.0 +particle_api
 x 2016-01-08 simplify tracer/field addition mechanism                                @design +v1.0 +particle_api
 x 2016-01-08 add stat choice parameter to add_particles                              @design +v1.0 +particle_api
+x 2016-01-15 particle output is broken when niter_part != 1                          @bugfix

todo.txt

-(A) particle output is broken when niter_part != 1                          @bugfix
 (B) document coordinate conventions                                         @documentation +v1.0
 (B) compute z polynomials only when needed                                  @optimization +v1.0
 (B) read https://www.xsede.org/documents/271087/369161/ExtScale-Koziol.pdf  @optimization @HDF5 +I/O