Determine correction for Greenwich sidereal time.
For old models, use just the projection of nutation in longitude to
correct mean to true sidereal time.
For 2000 and later models, we use GST-GMST = -(ERA-GST)-(GMST-ERA)
where ERA-GST= Equation of origins, and GMST-ERA was saved earlier
when the mean sidereal time at midnight was computed.   See
Wallace & Capitaine A&A 2006 and Capitaine, Wallace, & Chapront 2003

kind  =0 nutation-precession determined
kind  =1 nutation-precession and its derivative determined
nutpr =nutation times precession matrices
dnutpr=derivative of nutation times precession matrices
prec  =precession matrix
dprec =derivative of precession matrix
moblq,dmoblq=mean obliquity of ecliptic and its derivative
dpsi  =nutation in longitude and its derivative (radians)
deps  =nutation in obliquity and its derivative (radians)
(unit of time for derivatives is the second)

add corrections to nutation theory to account for the
earth's elasticity and fluid core
if mod(jct28,2).ne.0 apply melchior's corrections
to woolards's rigid-body theory
if jct(29).gt.0 apply corrections input as con(3-18)
note:  these two options are not mutually exclusive