Review default colours and style in ontopy/graph.py

[jesper-friis]

EMMO has to give all of them a unique default colour/style combination. One possibility could be to make mereological, topological and semiotical relations solid, dashed and dotted respectively. For colours we could either:

• leave it to the user to define them for each plot
• define explicit colours for some high-level/important relations within each category
• define a colour cycle and more or less randomly assign them to the relations present in the graph (pros: all relations will get a unique colour by default; cons: the same relation may get different colours in two different graphs)

It is also possible to support all three alternatives.

[francescalb]

I have now added so that if a defualt is not given for the given relation, the default style olive, solid is used. I think it would be smart to have an option where edgelabels are added only for those relations that do not hava a unique style.

[francescalb]

Closed with PR469