r. goldstein     april, 1976
this routine returns the first guess for the time delay for
a leg of the time delay iter. scheme. it uses a polynomial
extrapolation of the delays of up to three previous points
to accomplish this.  entries are made to tsav to save the
previous points and to dginit to initialize at the beginning
of a series.

delay - the returned value of the extrapolated time delay
in seconds
leg - the leg of the iteration
1 receive to reflect time
2 s/c a to s/c b time
3 reflect to send time
for only 1 spacecraft, use leg=1 and leg=3.
jdarg,utarg - time at which to calculate delay
(jdarg-days,  utarg-seconds)