PyBullet: allow to update an existing heightfield shape

Also, use flags = p.GEOM_CONCAVE_INTERNAL_EDGE to enable internal edge filtering for heightfield (disabled by default)
See https://github.com/bulletphysics/bullet3/blob/master/examples/pybullet/examples/heightfield.py

src/BulletCollision/CollisionShapes/btHeightfieldTerrainShape.h

@@ -220,6 +220,10 @@ public:
 	{
 		m_triangleInfoMap = map;
 	}
+	const unsigned char* getHeightfieldRawData() const
+	{
+		return m_heightfieldDataUnsignedChar;
+	}
 };
 
 #endif  //BT_HEIGHTFIELD_TERRAIN_SHAPE_H