Function wrappers for std::chrono time usage.

Placeholders converting from chrono::seconds to double
for animation. Will eventually replace all use of double with
std::chrono per gh #49.

src/choreograph/Channel.hpp

@@ -178,6 +178,10 @@ class Channel
   Channel(T value, Time at_time, Curve::Type curve_type, Keys&&... keys) {
     insertKey(value, at_time, curve_type, std::forward<Keys>(keys)...);
   }
+  template <typename ... Keys>
+  Channel(T value, Seconds at_time, Curve::Type curve_type, Keys&&... keys) {
+    insertKey(value, at_time, curve_type, std::forward<Keys>(keys)...);
+  }
 
   /// Return the value of the channel at a given time.
   T value(Time at_time) const;
@@ -202,6 +206,12 @@ class Channel
   template <typename ... Keys>
   Channel& insertKey(T value, Time at_time, Curve::Type curve_type, Keys&&... keys);
 
+  /// Insert a key at the given time using chrono::seconds.
+  Channel& insertKey(T value, Seconds at_time, Curve::Type curve_type = Curve::Linear) { return insertKey(value, at_time.count(), curve_type); }
+  /// Insert multiple keys using chrono::seconds.
+  template <typename ... Keys>
+  Channel& insertKey(T value, Seconds at_time, Curve::Type curve_type, Keys&&... keys);
+
   bool     empty() const { return _keys.empty(); }
   Time     duration() const { return empty() ? 0 : _keys.back().time; }
   const std::vector<Key>&   keys() const { return _keys; }
@@ -296,6 +306,15 @@ Channel<T>& Channel<T>::insertKey(T value, Time at_time, Curve::Type curve_type,
     return *this;
 }
 
+template <typename T>
+template <typename ... Keys>
+Channel<T>& Channel<T>::insertKey(T value, Seconds at_time, Curve::Type curve_type, Keys&&... keys)
+{
+    insertKey(value, at_time, curve_type);
+    insertKey(std::forward<Keys>(keys)...);
+    return *this;
+}
+
 template <typename T>
 typename Channel<T>::KeyManipulator Channel<T>::keyControl(size_t desired_index) {
   auto index = std::min(desired_index, _keys.size() - 1);

src/choreograph/TimeType.h

@@ -44,6 +44,7 @@ namespace choreograph
 ///
 
 using Time = double;
+using Seconds = std::chrono::duration<double, std::chrono::seconds::period>;
 
 /// Wrap \a time past \a duration around \a inflectionPoint.
 inline Time wrapTime( Time time, Time duration, Time inflectionPoint=0.0f )

src/choreograph/Timeline.h

@@ -75,6 +75,8 @@ class Timeline : public TimelineItem
   /// Add a cue to the timeline. It will be called after \a delay time elapses on this Timeline.
   TimelineOptions cue( const std::function<void ()> &fn, Time delay );
 
+  TimelineOptions cue( const std::function<void ()> &fn, Seconds delay ) { return cue( fn, delay.count() ); }
+
   //=================================================
   // Adding TimelineItems.
   //=================================================