Add dashboard to pip package

[cwmeijer]

Or make dashboard installable someway else.
Maybe it should get its own package or its subpackage like dianna[dashboard]

[stefsmeets]

#551 adds a cli command to start the dianna dashboard: dianna-dashboard

If we add the dashboard to the package resources, we can install it using `pip install dianna[dashboard], so that the dashboard works 'out-of-the-box'.