Feature: Terrain Preview and simple Elevation Query

[okimiko]

This PR will add

• a preview for terrain data (in addition to the current raw data preview)
  • highlight color is based on the encoding
• a simple elevation api based on served terrain data
  • includes a link on the index page
  • for details, see endpoints.rst

Both require configured encoding (see config.rst).

In general this adds just some eye candy and an "inspect" function for terrain like the geojson endpoint.

[okimiko]

Thanks for all suggestions so far. I did a local build and converted my terrain mbtile source to a pmtile and both are working so far :)

[acalcutt]

I was noticing how long some of the decimal point values are. I wonder if we should add a limit to the number of decimal places, maybe something like 7

[okimiko]

@acalcutt: That should be no problem, but at least for lat/long (float-url) the values are the same as the input, which are taken from leaflet click event. For x/y the values are calculated. Or should we only limit the display in the preview?

[acalcutt]

I mean more when it was displayed, like

But possibly the output should have a limit also