This is an attempt at solving issue #188.

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.

When only latitude and longitude are specified, the elevation is
automatically computed from the Digital Elevation Model.

The new name explains more clearly that several sensors can be set up at
once.

We now use a single position for all pixels (but of course still
different line-of-sight vectors), as it doesn't really make sense to
consider different positions as pixels are only a few microns away from
each other.

The current altitude must be considered when computing light time
correction. Previous computation was slightly wrong as it only computed
light time from the ellipsoid. This induced problems when computing
inverse correction.