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

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.

Measurement -> ObservedMeasurement
Evaluation      -> EstimatedMeasurement
EvaluationModifier -> EstimationModifier

These times scales may be used in SP3-c files.

The class acts as a general bridge between estimation algorithms like
orbit determination or maneuver optimizer and underlying physical
models.

This allows EvaluationModifier to change the weight during iterations,
for example to ignore outliers measurements after a few iterations (say
more than 5σ after 2 or 3 iterations).

As the measurements already know which parameters they depend on,
it didn't really make sense to pass them the parameters back from
the top level least squares estimator.