subr. hrtrp - j.f.chandler - 1979 nov
derived from subr. sctrp - j.f.chandler, 1977 may
this is a control routine for interpolating a body from its
integration (or n-body or s-body) tape - hermite method
input parameters:
jd      integer value of the coord. time of the desired pt
fract   fractional part of the coordinate time of the point
nvel =0 positions determined
1 velocities determined
-1 velocities determined from position y-vectors
lcntl=0 use results of last call if possible
1 do not use results (must recalculate y vectors)
-1 same as 0
icall   indicates which is the calling routine
1 - emtrp,  2 - mntrp,  3 - pltrp,  4 - sbtrp,
5 - sctrp,  6 -(prtrp), 7 -(ertrp)
yh      array for interpolation y-vectors
x       output coordinate array
body    coordinates from integration tape
hc      tabular intervals
jdtb    tabular julian dates
frtb    tabular fractions of a day
p       p-q array to be used

note for future work:
the trick of using the results of the last call is good only
within the light time iteration since velocity
calculations overwrite the y vectors.
however, velocities could be interpolated from the same
y-vectors, by numerical differentiation
(would require new function "dhermt")
and settling for (in effect) a lower-order polynomial