Form the equatorial to ecliptic rotation matrix (IAU 1980 theory). (double precision)
date double TDB (loosely ET) as Modified Julian Date (JD-2400000.5)
rmat double matrix
Murray, C.A., Vectorial Astrometry, section 4.3.
The matrix is in the sense v[ecl] = rmat * v[equ]; the equator, equinox and ecliptic are mean of date.
slaDeuler Defined in slamac.h: DAS2R P.T.Wallace Starlink 31 October 1993