Revision 124

Moved some processing from fxhyp_loop_ik to fxhyp. Now fxhyp_loop_ik
does not necessarily give longitudes near [-pi, pi]. In fxhyp, we look
in rlonm025 whether we need to move the array toward [-pi, pi]. If so,
we apply the same move to all grids: rlonm025, rlonv, rlonp025, rlonu
and the corresponding derivatives. The move itself is done by the new
procedure principal_cshift. This revision makes the logic
clearer. (For example, we do not have a saved variable is2 in
fxhyp_loop_ik any longer and we remove a test on ik in fxhyp_loop_ik.)

Fixed a bad error message in fxhyp_loop_ik: talked about rlonu when
xvrai is not always rlonu.