Jupyter Notebook does not delete figures consuming memory #1

When developing a model using InteractiveModel and IPWInteract
the figures are refreshed again and again after minor changes of the
InteractiveModel subclass. Previous figures remain in memory and are
not automatically deleted. One option to fix this is to restart the kernel, the other
is to explicitly close the figures. You can ge the current figure numbers using

plt.get_fignums()

Individual figures can be closed via

plt.close(plt.figure(1))

Automatic closure of the figure upon deletion of InteractiveModel is implemented
but is not called - must be coordinated with the garbage collection within the jupyter server.