The max_val parameter in HeatMap is still necessary

[karim-soufi]

Hello,

I think the max_val is still necessary.
For example if I want to generate two heatmaps with for two different dataframes, the calculated max would be different, hence I cant compare the both of them without having the same max_val being referenced.

[Conengmo]

This relates to #1271.

Looks like the max_val parameter actually doesn't exist in the underlying JS plugin! See https://github.com/Leaflet/Leaflet.heat#reference. The parameter is called max.

You can supply this parameter as a keyword argument now already:

HeatMap(max=1.0)

If this is an important parameter to have, we could add it to the class 'officially'. PRs welcome.

If somebody can report back on whether the max parameter actually works, that's helpful as well.