ENH: add Function1 wrapping for functionObject trigger

Returns a 0/1 value corresponding to function object trigger levels.

    Usage:
    \verbatim
        <entryName> functionObjectTrigger;
        <entryName>Coeffs
        {
            triggers        (1 3 5);
            defaultValue    false;  // Default when no triggers activated
        }
    \endverbatim

ENH: add reset() method for Constant Function1

ENH: allow forced change of trigger index

- the triggers are normally increase only,
  but can now override this optionally

src/OpenFOAM/db/functionObjects/functionObjectList/functionObjectList.C

@@ -509,7 +509,7 @@ Foam::autoPtr<Foam::functionObjectList> Foam::functionObjectList::New
 
 Foam::label Foam::functionObjectList::triggerIndex() const
 {
-    return propsDict().getOrDefault<label>("triggerIndex", labelMin);
+    return propsDict().getTrigger();
 }