This allows for example to retrieve the DEM elevation for a
latitude/longitude pair.

This can be used to set up min/max search lines.

This is still preliminary work, convergence seems slightly wrong.

This reverts commit b61d7ead.

This helps caller to set fix the min/max lines as we can provide
information about where the point is expected.

This algorithm is similar in spirit to the IGNORE_DEM_USE_ELLIPSOID, but
with a user-specified elevation instead of always using 0.0 elevation.

Implements feature #187.

Thanks to Espen Bjørntvedt for the translations.

The derivatives are computed with respect to transforms parameters.
This is the first step towards los calibration.

Thanks to Lucian Barbulescu for the translation.