this function computes ct - at for a given epoch using moyer's
formulation for annual and monthly terms (see tr 32-1527, p.14
from jpl), and an accurate( approx. 1 picosecond), but simple
diurnal term from d. robertson at mit 24-404.
the diurnal term suffers from a misconception.  asg 1977
the nominal variable part (rate) is saved in (d)ctatv
nterm=1  annual term only, site info not required
nterm=2  monthly and diurnal terms included
nterm=3  annual and monthly terms
nterm=4  integrated annual term, analytical monthly and diurnal
nterm=5  integrated annual term, analytical monthly
n=1      receive site
n=2      send site