Add a contant elevation algorithm
The IGNORE_DEM_USE_ELLIPSOID choice for selecting the intersection
does not allow selecting a non-zero elevation.
Either an additional choice should be available for constant elevation (which
would indeed not require a DEM either), or the existing algorithm should allow
to select a non-zero elevation.
(from redmine: issue id 187, created on 2015-01-14, closed on 2016-02-10)