Workaround for impossible elevation values

[eldang]

This implements the simplest approach I can think of to https://github.com/eldang/elevation_lookups/issues/13 , i.e.:

• If the first point in a line gets an impossibly negative elevation, skip that point and keep going until we get a plausible value for something.
• If there are no valid values for the whole line, just write a blank line to the output file instead of garbage.
• If later points in a line get impossibly negative elevations, just assume that their real elevation is the same as their predecessor.

[eldang]

Good point. I'll do that before merging this so I don't forget.