A class defining the ICRS coordinate system.
This should probably be the reference coordinate system, but it
differs only very slightly from the J2000 coordinates.
We are assuming the the J2000 coordinate system is FK5 based.
This may not be true for at the 50 mas level.
According to Feissel and Mignard (1998) the rotation angles for
the three axes are between the ICRS coordinates and FK5
(-19.9, -9.1, -22.9)