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/functionObjectProperties/functionObjectProperties.C

@@ -79,26 +79,38 @@ Foam::dictionary& Foam::functionObjects::properties::getObjectDict
 }
 
 
-bool Foam::functionObjects::properties::setTrigger
-(
-    const label triggeri
-)
+void Foam::functionObjects::properties::clearTrigger()
 {
-    label oldTriggeri = getOrDefault<label>("triggerIndex", labelMin);
-
-    if (triggeri > oldTriggeri)
-    {
-        set("triggerIndex", triggeri);
-        return true;
-    }
-
-    return false;
+    remove("triggerIndex");
 }
 
 
 Foam::label Foam::functionObjects::properties::getTrigger() const
 {
-    return getOrDefault<label>("triggerIndex", labelMin);
+    // Like getOrDefault, but without reporting missing entry (noisy)
+    label idx = labelMin;
+    readIfPresent("triggerIndex", idx);
+    return idx;
+}
+
+
+bool Foam::functionObjects::properties::setTrigger
+(
+    const label triggeri,
+    bool increaseOnly
+)
+{
+    const label currTriggeri = getTrigger();
+
+    if (increaseOnly ? (triggeri > currTriggeri) : (triggeri != currTriggeri))
+    {
+        set("triggerIndex", triggeri);
+        return true;
+    }
+
+    // TBD: any special handling for triggeri == labelMin - eg, clearTrigger()
+
+    return false;
 }