[FEATURE][API] Add iterator for QgsGeometryCollection

Iterates over the geometries in the collection, allowing this type
of code:

  gc = QgsGeometryCollection()
  gc.fromWkt('GeometryCollection( Point(1 2), Point(11 12), LineString(33 34, 44 45))')
  for part in gc:
    print(part.asWkt())

Author: nyalldawson

python/core/auto_generated/geometry/qgsgeometrycollection.sip.in

@@ -270,6 +270,16 @@ corresponds to the last geometry in the collection.
     }
 %End
 
+    SIP_PYOBJECT __iter__() /TypeHint="QgsGeometryPartIterator"/;
+%Docstring
+Iterates through all geometries in the collection.
+
+.. versionadded:: 3.6
+%End
+%MethodCode
+    sipRes = sipConvertFromNewType( new QgsGeometryPartIterator( sipCpp ), sipType_QgsGeometryPartIterator, Py_None );
+%End
+
     virtual QgsGeometryCollection *createEmptyWithSameType() const /Factory/;
 
 

src/core/geometry/qgsgeometrycollection.h

@@ -277,6 +277,16 @@ class CORE_EXPORT QgsGeometryCollection: public QgsAbstractGeometry
       sipIsErr = 1;
     }
     % End
+
+    /**
+     * Iterates through all geometries in the collection.
+     *
+     * \since QGIS 3.6
+     */
+    SIP_PYOBJECT __iter__() SIP_TYPEHINT( QgsGeometryPartIterator );
+    % MethodCode
+    sipRes = sipConvertFromNewType( new QgsGeometryPartIterator( sipCpp ), sipType_QgsGeometryPartIterator, Py_None );
+    % End
 #endif
 
     QgsGeometryCollection *createEmptyWithSameType() const override SIP_FACTORY;

tests/src/python/test_qgsgeometry.py

@@ -498,6 +498,12 @@ def testGeometryCollectionPythonAdditions(self):
         with self.assertRaises(IndexError):
             del g[-3]
 
+        # iteration
+        g = QgsGeometryCollection()
+        self.assertFalse([p for p in g])
+        g.fromWkt('GeometryCollection( Point(1 2), Point(11 12), LineString(33 34, 44 45))')
+        self.assertEqual([p.asWkt() for p in g], ['Point (1 2)', 'Point (11 12)', 'LineString (33 34, 44 45)'])
+
     def testReferenceGeometry(self):
         """ Test parsing a whole range of valid reference wkt formats and variants, and checking
         expected values such as length, area, centroids, bounding boxes, etc of the resultant geometry.