Update richness figure colors

[jcoliver]

Two things to try/consider:

• [x] Use a different color palette (currently low is yellow and high is green). Try the inferno palette as a first start (often used by eBird).
• [x] For areas where we have predictions and the predicted number of species is zero, use a markedly different color.
• [ ] (stretch) Consider adding polygon(s) around those areas with >=4 species, since we talk about these areas in manuscript.

[ezylstra]

Updated colors. Used the eBird abundance/inferno palette for richness, with a light gray for areas with 0 species. Used a eBird-like palette for the richness change figures, with blue for positive values and red for negative values.

The 3rd item (denoting "hotspots") proved a bit difficult. There are lots of very small hotspots, so using a polygon to delineate those areas wasn't possible. Alternatively, we talked about binning values, having one color for 0 spp, one for 1-3 spp, and one for 4-7 spp. However, I didn't like that either and we're essentially throwing out information. Maybe this isn't necessary or if so, could put in supplement.

[jcoliver]

Closing for now; polygons around high richness areas did not add to figure.