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

USNO has confirmed there was an issue in this bulletin A. We can
therefore consider the behaviour is not expected and later bulletin A
published in the same conditions (i.e. with prediction section starting
just the day of a leap second) won't have an off-by-one value in
reference TAI-UTC.

Fixes issue #194.

The code is complete, but not tested at all yet!

Add testcase for calculating the GeomagneticFieldElements based on a height above MSL using the Geoid.

Update GeoMagneticField unit tests: use latest versions of magnetic models - IGRF12 and WMM2015, fix computed reference values - use height above ellipsoid instead of mean sea level.

Fix javadoc of GeoMagneticField#calculateField() method: altitude is actually the height above the WGS84 ellipsoid. Pending are updates to the tests as some of the reference values have been computed with the wrong altitude, although the actual difference is quite small it should be fixed for consistency.

Added a Geoid BodyShape that computes the harmonic continuation Geoid. Also
added ReferenceEllipsoid which extends OneAxisEllipsoid to include ellipsoidal
gravity. The EarthShape interface is useful when using higher order Earth models
so that one can always obtain the underlying ellipsoid.

Tests for Geoid and ReferenceEllipsoid are also included, along with some test
support code in OrekitMatchers.

Fixes issue #191.

The target needs to be on body surface as the class is a specialization
of GroundPointing and could be used in YawCompensation mode. The target
point is now properly on surface, even if the line from spacecraft to
target also contains the body center, which is not the target.

Fixes issue #100.

Added two constructors that only take position and velocity and set the
acceleration to zero. Also added test cases.