planner_multi: add != operator

Problem: `planner_multi` equality testing needs a convenience !=
operator.

Add a `planner_multi` `!=` operator.

Author: milroy

resource/planner/c++/planner_multi.cpp

@@ -178,6 +178,11 @@ bool planner_multi::operator== (const planner_multi &o) const
     return true;
 }
 
+bool planner_multi::operator!= (const planner_multi &o) const
+{
+    return !operator == (o);
+}
+
 void planner_multi::erase ()
 {
     if (!m_planners.empty ()) {

resource/planner/c++/planner_multi.hpp

@@ -28,6 +28,7 @@ class planner_multi {
     planner_multi (const planner_multi &o);
     planner_multi &operator= (const planner_multi &o);
     bool operator== (const planner_multi &o) const;
+    bool operator!= (const planner_multi &o) const;
     void erase ();
     ~planner_multi ();