Use elevation data

[dabreegster]

• Decide how to access elevation data for dynamically imported areas -- COG?
• Calculate a gradient per road segment
• Show this detail in the web app as a layer
• Use Tobler's hiking function for the walk profile
• Let users customize the base walking speed

[dabreegster]

What pure Rust COG readers are there? https://github.com/developmentseed/async-tiff looks the closest to working, context at georust/geotiff#13.

This might be easier to do with pre-building bundled files.