ipa: libipa: Allow creation of empty Histogram

For convenience's sake allow the creation of empty Histograms so
they can be embedded within other Classes and filled out with
data at some later point in time.

Reviewed-by: Stefan Klug <stefan.klug@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/ipa/libipa/histogram.cpp

@@ -28,6 +28,15 @@ namespace ipa {
  * specified bin. It can be used to find quantiles and averages between quantiles.
  */
 
+/**
+ * \fn Histogram::Histogram()
+ * \brief Construct an empty Histogram
+ *
+ * This empty constructor exists largely to allow Histograms to be embedded in
+ * other classes which may be created before the contents of the Histogram are
+ * known.
+ */
+
 /**
  * \brief Create a cumulative histogram
  * \param[in] data A pre-sorted histogram to be passed