Add AtLeastOneAmong rule

src/jflow/rules.py

@@ -244,3 +244,20 @@ class ForbiddenChars(ValueRule):
         for value in self.values_arg:
             if value in self.parameter_value:
                 self.error("Character not allowed: \"" + value + "\"")
+
+
+class AtLeastOneAmong(LinkRule):
+    """
+    At least one parameter among the list must be set
+    """
+
+    def check(self):
+        if self.parameter_value is None:
+            has_one = False
+            for grp_param in self.targets_args:
+                if not getattr(self.wf_instance, grp_param).is_None:
+                    has_one = True
+                    break
+            if not has_one:
+                all_required = [self.parameter_name] + self.targets_args
+                self.error("Please give at least one of these options:\n\t--" + "\n\t--".join(all_required))