Add hasStarted() const to WallTimer and SteadyTimer API (ros#1565)

* roscpp: copy hasStarted() member function from ros::Timer to ros::WallTimer and ros::SteadyTimer

ros::Timer::hasStarted() has been added in ros#1464. The same member function should exist in the other
two timer implementations, too, for completeness.

* Check for nullptr in WallTimer::hasStarted() and SteadyTimer::hasStarted()

Analogous to fe9479c (ros#1541).

clients/roscpp/include/ros/steady_timer.h

@@ -67,9 +67,11 @@ class ROSCPP_DECL SteadyTimer
 
   /**
    * \brief Set the period of this timer
+   * \param reset Whether to reset the timer. If true, timer ignores elapsed time and next cb occurs at now()+period
    */
   void setPeriod(const WallDuration& period, bool reset=true);
 
+  bool hasStarted() const { return impl_ && impl_->hasStarted(); }
   bool isValid() { return impl_ && impl_->isValid(); }
   operator void*() { return isValid() ? (void *) 1 : (void *) 0; }
 
@@ -97,6 +99,7 @@ class ROSCPP_DECL SteadyTimer
     Impl();
     ~Impl();
 
+    bool hasStarted() const;
     bool isValid();
     bool hasPending();
     void setPeriod(const WallDuration &period, bool reset=true);

clients/roscpp/include/ros/wall_timer.h

@@ -71,6 +71,7 @@ class ROSCPP_DECL WallTimer
    */
   void setPeriod(const WallDuration& period, bool reset=true);
 
+  bool hasStarted() const { return impl_ && impl_->hasStarted(); }
   bool isValid() { return impl_ && impl_->isValid(); }
   operator void*() { return isValid() ? (void*)1 : (void*)0; }
 
@@ -98,6 +99,7 @@ class ROSCPP_DECL WallTimer
     Impl();
     ~Impl();
 
+    bool hasStarted() const;
     bool isValid();
     bool hasPending();
     void setPeriod(const WallDuration& period, bool reset=true);

clients/roscpp/src/libros/steady_timer.cpp

@@ -125,6 +125,11 @@ SteadyTimer::Impl::~Impl()
   stop();
 }
 
+bool SteadyTimer::Impl::hasStarted() const
+{
+  return started_;
+}
+
 void SteadyTimer::Impl::start()
 {
   if (!started_)

clients/roscpp/src/libros/wall_timer.cpp

@@ -42,6 +42,11 @@ WallTimer::Impl::~Impl()
   stop();
 }
 
+bool WallTimer::Impl::hasStarted() const
+{
+  return started_;
+}
+
 void WallTimer::Impl::start()
 {
   if (!started_)