add outcomes png to dashboard

[dwrensha]

Adds the output of outcomes_to_images.py to the dashboard page.

Makes generate_dashboard.py responsible for launching subprocesses for whatever intermediate artifacts it needs. This way, the dashboard generating in blueprint.yml is reduced to a single line that calls generate_dashboard.py.

[teorth]

These are intriguing visual artefacts in the graph. Some periodicity and some extra structure near the diagonal. Not sure what to make of all that!