Parametrizable color scheme for graphs

[hcarolus]

Is your feature request related to a problem? Please describe.
The module puts out great graphs. However, we'd like to make the colors parametrizable.

Describe the solution you'd like
Add the color scheme as a member variable to ComputationGraph. To apply a different color scheme, set it via a parameter in the function draw_graph.

Describe alternatives you've considered
None