add function for measuring gradient at a given location over given interval

[smnorris]

Something like this:

fwa_gradient(blue_line_key, measure, length)

Measure gradient along stream given by blue_line_key, from (measure - length / 2) to (measure + length / 2).

Relatively straightforward except:

1. handle locations where location to extract elevation is outside of stream endpoints
2. 1 is straightforward for most streams, but on cross-border streams the query gets awkward

Just implement with 1 for now.