Add heatmap visualisation of active fire points

[ajgeers]

When displaying many points close together on a map - for example when zooming out to view the active fire data of a whole continent - point markers might start overlapping and it can sometimes be difficult to correctly interpret how points are distributed. Heatmaps are an alternative visualisation technique that better highlight the overall distribution rather than showing individual points.

Use the Leaflet.heat plugin to visualise the active fire points with a heatmap. Add a layer control to switch between point marker and heatmap visualisations.

Here is an example heatmap showing the distribution of earthquakes (source):

[ajgeers]

The Leaflet.heat library requires the point data to be in the following format:

[
    [50.5, 30.5, 0.2], // lat, lng, intensity
    [50.6, 30.4, 0.5],
    ...
]

This issue discusses how to convert a GeoJSON to this format.