Improvement for tutorial 'Unconstrained shape design of a two way mixing channel'

[JoostvanRaak]

In step 4: Gradient Validation, one has to run the postprocess.py script which takes doe.his as input. The previous script, gradient_validation.py however gives doe.csv as an output. So instead of copying to it to the .his extension, gradient_validation.py could perhaps also write directly to a history file.

[bigfooted]

This will be fixed in #49