Fixes issue #227

Thanks to Michael Turner for the initial patch.

Github: closes #7

Filtering is based on an user-provided enabling predicate function. This
allow for example to dynamically turn some events on and off during
propagation or to set up some elaborate logic like triggering on
elevation first time derivative (i.e. one elevation maximum) but only
when elevation itself is above some threshold.

Works for anomaly, latitude argument, or longitude argument, either
true, mean or eccentric.

Fixes issue #213.

Derivatives continuity at both endpoints of the transition can be forced
to match rotation, rotation rate and rotation acceleration.

Fixes issue #6.

Fixes issue #206.

    
Fixes issue #115.

This support is in addition to the legacy support for IERS
UTC-TAI.history file which is still supported and also enabled by
default.

Thanks to Lucian Barbulescu for providing the translation.

The Eckstein-Hechler propagator now generated Cartesian orbits, with
velocity computed to be fully consistent with model evolution. A side
effect is that if users rebuild circular parameters from the generated
orbits, they will generally not math exactly the input circular
parameters (but position will match exactly).

Fixes issue #180.

Bulletin A provides rapid service and prediction, whereas bulletin B
provides final values. Bulletin A are suitable for real-time operation
and bulleting B more oriented towards a posteriori accurate analysis.

Thanks to Ioanna Stypsanelli!

This was the last par still in apt format. Now everything uses markdown.