Add cluster features to scattermapbox trace

[elben10]

I've added a trace that allow one to use mapbox clustering layer.

Currently it doesn't support the fill, selection and text features.

[archmoj]

@elben10 Thanks for the PR!
Could you please pick this commit d9c6e02 that fixes the mapbox filename, adds the baseline and some tests.

[alexcjohnson]

@elben10 thanks for the PR, it looks really useful! But let's take a step back before going further with it - does this really need to be a separate trace type? It seems to me as though, in terms of the attributes involved, it's just a few extra options on top of scattermapbox to control the clustering, and the attributes regarding lines are not allowed. I'd suggest adding a cluster.enabled attribute, which defaults to true whenever any of the other cluster attributes is set explicitly; then when cluster.enabled===true the only mode we will allow is markers and the line attributes will automatically not be used. Then in the following steps we can just look at the cluster.enabled attribute to determine which pathway to take.

Does that seem like a reasonable way to include this feature?

[elben10]

@alexcjohnson I think that sounds like a better choice. I'll update the code accordingly

[archmoj]

337 file changes! This is now extremely hard to review.
Could you start a fresh PR and rebase/squash the changes you made to mapbox only on top of v1.58.1?
Please also consider giving us as the maintainers permission to edit as described in the PR template.
Thank you!

[archmoj]

@elben10
To make it easier for you to submit a new PR I fixed the merge conflicts on this temporary branch:
https://github.com/plotly/plotly.js/compare/cluster-scattermapbox

To open the final PR you may consider starting a new fork.
Then

git checkout cluster-scattermapbox
git reset HEAD^
git add .
git commit 

So that you get credit for the great contribution you made.
Then

git push -f

Then please open a PR from your cluster-scattermapbox branch to our master.

Does that make sense?

[elben10]

Yes it does. The pull request is now created! Thanks for the help and guidance!