update todo

todo.txt

+(A) check divfree function
 (B) FFTW interpolator doesn't need its own field            @optimization +v1.0
 (B) compute z polynomials only when needed                  @optimization +v1.0
-(B) use less memory                                         @optimization
+(B) see if HDF5 performance can be improved                 @optimization +I/O
 (B) split library into core and extra                       @optimization +v1.0
+(B) use less memory                                         @optimization
 (C) clean up machine_settings mess                          @design @documentation +v2.0
 (C) code overview                                           @documentation
 (C) move stat I/O to cpp lib                                @design @HDF5
 (C) set up mechanism for adding in new PDEs                 @design +v2.0 +alternate_algorithms
 (C) test involving hydrodynamic similarity                  @tests
 (C) use HDF5 io for fields                                  @design @HDF5 +I/O
-(D) tests should not overwrite other tests (tox_full)       @tests
 (D) generalize interpolation comparison test                @tests
 (D) test anisotropic grids                                  @tests
 (D) test non-cubic domains                                  @tests
+(D) tests should not overwrite other tests (tox_full)       @tests
 (E) add u-equation algorithm for testing purposes           @tests +alternate_algorithms
 (E) pure python DNS addon: pros and cons                    @tests +alternate_algorithms
 (F) add switch to turn off simulation