Add SHAP maps for location and proximity features

[Damonamajor]

This pull request produces a rendered leaflet map that demonstrates the neighborhood-level impact of the 20 largest indicators on the mean(absolute value) of housing.

• There is likely a way to link the max_display = 25L which is present throughout the SHAP doc to ensure that the same variables are shown throughout.
• When rendering and emailing, the leaflet does not retain its self-contained attributes. Using saveWidget(shap_map, file ="shap_map.html", selfcontained = TRUE) produced a widget that could be properly emailed, but I was then not able to include it in the doc.

[dfsnow]

@Damonamajor This is looking pretty close. I was able to build it on the server then download it locally and run it. I'm not sure why the version you emailed me wasn't self-contained.

Two minor points of feedback so far:

• Change the tile base layer to Carto's positron
• Let's try coloring the nbhds by the relative importance of each feature, rather than the mean absolute SHAP. Mean abs. SHAP alone is weird because more expensive places will have higher SHAP values by default.
• Drop the dashed polygon boundaries. Use either solid line or no boundaries