Thanks to Guylaine for finding the issues!

Fixes #505

Fixes #494

Having a single propagator in the Scheduler does not fulfills
the need for inter satellites measurements.

The number is unstable.

This adaptor allows wrapping an existing detector while changing its
behaviour.

Fixes #495

Fixes #486

Fixes #496

Since OrekitException is unchecked GFunction can be deprecated in favor of
ToDoubleFunction.

Fixes #502

The predicates by themselves are simple but can be combined to
match mission-specific or measurement-specific needs. As an
example an optical tracking of satellites on the stars background
using a telescope requires to have at measurement time the satellite
visible from the telescope location, the telescope location be
in the night side of the Earth and the satellite being still sunlit.
This can be described as a combing scheduling predicate that is the
AND combination of VisibleFromGround, GroundAtNight and Sunlit
basic predicates.