m.e.ash    nov 1969    subroutine ertpar
calculate partials w.r.t. precession,obliquity and et-ut1 paramtrs
f.a.kreimendahl   nov 1979   cleaned up and reno'd
partials w.r.t. annual wobble terms added

erotat con's 1-21 are shared by two mutually-exclusive sets
of parameters.  check to see which applies:
jct(29)=0
1- 6:  ad hoc small rotations and rates
7-10:  open
11-13:  polynomial in ct-ut2 or a1-ut1--see jct(34)
14-21:  seasonal terms in ct-ut2 or a1-ut1--see ict(34)
jct(29)>0
1- 2:  free core nutation (nearly-diurnal free wobble)
3-10:  in-phase components of adjustable nutation terms
14-day, 183-day, 365-day, & 18.6-yr
11-18:  out-of-phase components of adjustable nutation t
19-20:  (k/c) for fortnightly & monthly ut1 terms
21:  open