SRTM lookups now work across integer lat/lon lines

[eldang]

In this PR:

• SRTM is no longer explicitly configurable as a datasource. Instead it's the global fallback any time that none of the explicitly configured datasources cover the area we need.
• SRTM is downloaded as 1x1 degree tiles, breaking on integer lines of latitude & longitude because that seems to be the most robust.
• If an input file spans more than one of those files, rasterio.merge is used to combine them
• Also some minor documentation housekeeping

Tagging Anchorage (14,121 paths, spans across -150 longitude) takes 1m30s on my machine.

Work for the rest of this week, unless @dabreegster thinks these no longer make sense:

1. Parallelising lookups
2. Turning CONTRIBUTING.md into a usable document

Possible later steps:

• Do we still need to figure out a workaround for bridges? Probably better to discuss this offline; may depend on how raster data looks for Seattle.

[eldang]

After opening this PR I investigated an apparent free lunch I was getting, and it was of course no such thing. Closing to fix the bug; will reopen when fixed.

[dabreegster]

Sorry for the slow response, few more things came up. I'll try and get the SRTM data validated soon and see how the bridges turn out -- but it might be Tues or Wed now. Your plan for the rest of the week looks good, regardless!

[dabreegster]

LGTM code-wise. I'm rebuilding my local Docker container and running the import to compare contours, SRTM, and LIDAR.