Gradient scale colors for leaflet

[hmrtn]

Replace (or parameterize) AQI colors with continuous color scale to pas_leaflet()

[jonathancallahan]

Let's be careful about applying colors here.

The color scales for temperature, humidity and pwfsl_closestDistance are carefully constructed color palettes with breaks specific to each variable.

These color scales should be accessible as named palettes:

• "temperature"
• "humidity"
• "distance"

and should be the default for these variables.