Add support for extra traces

[almarklein]

• [x] Adds a extra_traces prop, which is a dcc.Store that users can write a list of traces to.
• [x] Adds example to show a contour this way.

I considered using pattern matching, which would make it possible for the application to define multiple stores for additional traces. However, by simply proving a store via a property, we make the simpler cases easier, and the more complex cases can still define multiple stores and then add a callback to aggregate these into the slicer.extra_traces.

Possible alternative names for extra_traces could be e.g. user_traces, `more_traces.

[emmanuelle]

Thanks! The checklist above mentions that you added an example showing a contour but I could not find it, did you maybe forget to add it?

[emmanuelle]

Regarding the name, I like both extra_traces and user_traces (not so much more_traces). Maybe a slight preference for extra_traces ?

[almarklein]

[...] did you maybe forget to add it?

Yes I did :)

[...] Maybe a slight preference for extra_traces ?

Let's just stick with extra_traces then.

[emmanuelle]

Just a color-related nitpicking comment, but it looks good to go!