Revision 112

Removed 8 first arguments of fxyhyper, use variables of module serre
instead.

Moved reading of variables of module serre from procedure conf_gcm to
new procedure read_serre.

In guide, added conditions to avoid useless calls to tau2alpha and
writefield. Bugfix: offline corresponds to alpha = 1. Open only one
NetCDF file to read number of vertical levels.

In tau2alpha, added conditions to avoid useless computations of dxdyu
and dxdyv. gamma is not needed for a regular grid.