Fix heatmap intensity

[Conengmo]

Closes #1271.

Weights are broken in the heatmap plugin. See the linked issue for more details.

The leaflet.heat dependency seems abandoned. There is a PR with a fix available, but it's not getting merged. Host this dependency in our repo and include the fix.

todo

• put in a userwarning that max_val is deprecated and why
• add a selenium test that verifies behavior with weights

[Conengmo]

Do not merge until the JS dependency URL has been updated after review has finished.

[Conengmo]

@ocefpaf could you take a look at this? The issue poster cannot help review it. It's a pretty bad bug though, and has been for some time (#1271, #496)

[ocefpaf]

LGTM but I have to confess I've been away from the code base for so long that I would like a regression test to be in place. However, do not let that prevent you from merging this as is.

[Conengmo]

Thanks for checking. About regression tests, how do you feel about having Selenium tests? I have time to add those. But maybe you can say something about making it work in Appveyor?

[ocefpaf]

how do you feel about having Selenium tests?

Those are great!

I have time to add those.

Awesome!

But maybe you can say something about making it work in Appveyor?

Just add a pytest mark and skip them on AppVeyor at the moment. I'll investigate later how to run them there. (Or move to AzurePipelines.)