Revision 115

Extracted code from tau2alpha for first call into new procedure
init_tau2alpha. dxdys, dxdyu, dxdyv are now local variables if guide
computed by init_tau2alpha. This allows us to remove terrible argument
type of tau2alpha: we just give to tau2alpha the right dxdy and
rlat.

In module conf_guide_m, changed default values of tau_min_*, because
0.02 is too small for the default daystep = 240, iperiod = 5. Changed
default values of guide_[uv] to false. Moved variable ok_guide from
conf_gcm_m to conf_guide_m, ok_guide is no longer an input parameter,
it is computed from guide_*. Had then to move test on ok_guide and
day_step from conf_gcm_m to conf_guide_m. Added checks on input
nudging parameters in procedure conf_guide. Upgraded variable factt to
module conf_guide_m in order to check nudging parameters. Bug fix:
variable guide_q was not in namelist conf_guide_nml.

Removed unused variables aire_min, aire_max of MODULE guide_m.

Moved the call to conf_guide from guide to gcm. This was needed to
define ok_guide before getting into guide (since ok_guide is no longer
in conf_gcm_m). Moved test on grossismx and grossismy from tau2alpha
to guide. It is clearer now that only tau_max is used for a regular
grid, and we do not have to repeat this test in each call to
tau2alpha. In guide, we only call writefield when alpha is not a
constant.