Provide a base class version of QgsFeatureSink::addFeature

Means that subclasses are only required to implement addFeatures

python/core/qgsfeaturesink.sip

@@ -24,7 +24,7 @@ class QgsFeatureSink
 
     virtual ~QgsFeatureSink();
 
-    virtual bool addFeature( QgsFeature &feature ) = 0;
+    virtual bool addFeature( QgsFeature &feature );
 %Docstring
  Adds a single ``feature`` to the sink.
  \see addFeatures()

python/core/qgsvectordataprovider.sip

@@ -197,7 +197,6 @@ class QgsVectorDataProvider : QgsDataProvider, QgsFeatureSink
      */
     virtual void enumValues( int index, QStringList& enumList /Out/ ) const;
 
-    virtual bool addFeature( QgsFeature &feature /In,Out/ );
     virtual bool addFeatures( QList<QgsFeature> &flist /In,Out/ );
 
     /**

src/core/qgsfeaturesink.cpp

@@ -17,6 +17,17 @@
 
 #include "qgsfeaturestore.h"
 
+bool QgsFeatureSink::addFeature( QgsFeature &feature )
+{
+  QgsFeatureList features;
+  features << feature;
+  bool result = addFeatures( features );
+
+  // need to update the passed feature reference to the updated copy from the features list
+  feature = features.at( 0 );
+  return result;
+}
+
 bool QgsFeatureSink::addFeatures( QgsFeatureIterator &iterator )
 {
   QgsFeature f;

src/core/qgsfeaturesink.h

@@ -41,7 +41,7 @@ class CORE_EXPORT QgsFeatureSink
      * \see addFeatures()
      * \returns true in case of success and false in case of failure
      */
-    virtual bool addFeature( QgsFeature &feature ) = 0;
+    virtual bool addFeature( QgsFeature &feature );
 
     /**
      * Adds a list of \a features to the sink.

src/core/qgsvectordataprovider.cpp

@@ -495,11 +495,6 @@ QVariant QgsVectorDataProvider::aggregate( QgsAggregateCalculator::Aggregate agg
   return QVariant();
 }
 
-bool QgsVectorDataProvider::addFeature( QgsFeature &feature )
-{
-  return addFeatures( QgsFeatureList() << feature );
-}
-
 void QgsVectorDataProvider::clearMinMaxCache()
 {
   mCacheMinMaxDirty = true;

src/core/qgsvectordataprovider.h

@@ -248,7 +248,6 @@ class CORE_EXPORT QgsVectorDataProvider : public QgsDataProvider, public QgsFeat
      */
     virtual void enumValues( int index, QStringList &enumList ) const { Q_UNUSED( index ); enumList.clear(); }
 
-    virtual bool addFeature( QgsFeature &feature ) override;
     virtual bool addFeatures( QgsFeatureList &flist ) override;
 
     /**