Revision 109

Moved a call to writefield from guide to tau2alpha. (dxdys does not
change with itau.) So dxdys does not need to be a module variable any
longer. Other variables of modules tau2alpha_m downgraded to local
variables of tau2alpha, since they were not used elsewhere.

Procedures write_field[13]d and formcoord were never called. Could
then remove int2str.

Inline writefield_gen into writefield.

CreateNewField takes an integer array argument instead of 3 scalar
integers. CreateNewField now creates a number of dimensions adapted to
the rank of the output field, instead of always 4 dimensions.

Changed names of variables of module write_field: fieldid to
ncid, fieldindex to record, fieldvarid to varid.

In writefield_gen, if index == -1, no need to call GetFieldIndex
again, we know that the result is nbfield.

In guide, moved calls to writefield for some variables inside if
first_call: those variables do not change with time. Removed ztau:
computed only to be output, does not seem meaningful. Removed
writefield for aire: does not change with time and is already in
"grilles_gcm.nc".