add validation upon mandatory options function

tiramisu/config.py

@@ -536,5 +536,11 @@ def make_dict(config, flatten=False):
             pass # this just a hidden or disabled option  
     options = dict(pathsvalues)
     return options
-# ____________________________________________________________
+
+def mandatory_warnings(config):
+    for path in config.getpaths():
+        try:
+            value = getattr(config, path)            
+        except MandatoryError:
+            yield path