made precipitation input optional to allow Original Hargreaves method

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@47687 15284696-431f-4ddb-bdfa-cd5b030d7da7

imagery/i.evapo.mh/main.c

@@ -90,6 +90,7 @@ int main(int argc, char *argv[])
     input_TEMPKMAX->description = _("Name of input maximum air temperature raster map [C]");
 
     input_P = G_define_standard_option(G_OPT_R_INPUT);
+    input_p->required = NO;
     input_P->key = "precipitation";
     input_P->label =
 	_("Name of precipitation raster map [mm/month]");