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.

In some cases, the line of sight has two intersections with a given
iso-latitude cone, and the selecting the closes one to spacecraft was
not always the appropriate choice. An hint from the caller was needed,
as a point close to the desired intersection.

It works in simple cases (no light time correction and no aberration of
light correction), and is still really slow.