Added check against primary_key and uniq

As a primary_key field has to be unique, added a check at DataHandler
instanciation to make sure a field set as a pk is not set as non-unique

lodel/leapi/datahandlers/base_classes.py

             del(kwargs['default'])
         for argname, argval in kwargs.items():
             setattr(self, argname, argval)
+            
+        if self.is_primary_key() and not self.is_uniq():
+            raise LodelException("'primary_key' field has to be uniq")
 
     ## Fieldtype name
     @classmethod
 
     def is_primary_key(self):
         return self.primary_key
+    
+    def is_uniq(self):
+        return self.uniq
 
     ##@brief checks if a fieldtype is internal
     # @return bool