emtrp - dec. 1975 - r. goldstein
velocities from numerical differentiation added  (nvel= -1)
by j.f.chandler, 1977 may
this is a control routine for interpolating the earth-moon
barycenter from its integration tape
input parameters:

idumm     ignored
jdt       integer value of the coord. time of the desired
point
frt       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 use old results only if jdt.frt is in same tab pt.
ip    = 1 use recieve arrays
ip    = 2 use send arrays
output is put in the xem(1-6,ip) vector

entry point 'etrp' is identical, except that it also calls
mntrp for the same epoch and stores the earth position in
'xemlsc(1-6,ip)' - control ixctl governs whether this
output is scaled:
ixctl=1 leave units as au, au/day
=2 convert to lt.sec., lt.sec./sec.
=3 compute position of site 'ip' w.r.t. sun