i n p u t   p a r a m e t e r s
nvel  =0 tide only
=1 tide and tide rate
gm  big g * mass(perturber)
g  local gravity acceleration
cphi  cosine(site latitude)
h  love number - vertical
l  love number - horizontal
r  vector from c.o.m. body to perturber's center
a  vector from c.o.m. body to site
dadr  vectors of partial derivatives of the
dadphi    site vector w.r.t. body-fixed
dadlam    spherical coordinates. (phi=latitude)

r e t u r n   p a r a m e t e r s
x  tide vector (and rate if nvel=1)
dxdh  partials of tide vector
dxdl   w.r.t. love numbers (6-vector if nvel=1)