a     - earth-fixed cartesian coordinates of the site
cphi  - cos(latitude) of site
fract - UTC fraction of a day of the desired epoch
t     - coordinate time from J2000.0 in Julian centuries
daidr - vectors of partial derivatives of the
daidphi    inertial site vector w.r.t. body-fixed
daidlam    spherical coordinates. (phi=latitude)
on return
xi    - correction to site position in inertial equatorial system
delr  - correction to site position in
delphi   local body-fixed
dellam   frame

Based on DEHANTTIDEINEL, ST1IDIU, ST1ISEM, and ST1L1 from the
International Earth Rotation and Reference Systems Service (IERS)
Conventions software collection.  The calculation is completely
reorganized to segregate the offsets by perturber, rather than by
type of term, and calculations are parametrized.  As such, this
routine is a functional alternative to VETIDE with almost the same
calling sequence, the differences being: instead of the local
acceleration of gravity at the site, this routine expects G*M and
mean radius, and it also expects both degree-2 and degree-3 Love
numbers.

This subroutine calculates the tidal displacement due to the
perturbation of the sun or moon, given the position vectors
of the perturber (in inertial coordinates) and the earth site (in
corotating body-fixed coordinates) and the transformation
between body-fixed and inertial coordinates.

I N P U T   P A R A M E T E R S
nvel  =0 tide only
=1 tide and tide rate (not implemented, returns zero rate)
gm  big g * mass(perturber)
g  big g * mass(body)
cphi  cosine(site latitude)
h2,h3  love numbers - vertical
l2,l3  love numbers - horizontal
ri  vector from c.o.m. body to perturber's center (inertial)
a  vector from c.o.m. body to site (corotating)
menrad  mean radius of body
daidr  vectors of partial derivatives of the
daidphi    inertial site vector w.r.t. body-fixed
daidlam    spherical coordinates. (phi=latitude)

R E T U R N   P A R A M E T E R S
xi  tide vector (inertial)
dxidh  partials of tide vector (inertial)
dxidl   w.r.t. degree-2 love numbers
delr  tidal offsets divided by love number
delphi   in local body-fixed
dellam   frame

sphi = a(3)/a1
slam = a(2)/cphi/a1
clam = a(1)/cphi/a1
transformation from topocentric to equatorial in corotating system

tidal terms, amplitudes in millimeters
fundamental arguments:
1 s   (1 month)
2 h   (1 year)
3 p   (8.8 years)
4 zns (18.6 years)
5 ps  (20936 years)