- len(QgsCurve) returns number of points in curve
- raise IndexErrors when calling pointN, xAt, yAt, zAt, mAt, setXAt, setYAt,
setMAt, setZAt with invalid vertex indices
- Add [] getter for retrieving specific vertices, eg. ls[0] returns QgsPoint(...)
- Add [] setter for setting specific (existing) vertices, e.g. ls[1] = QgsPoint(1,2)
- Add del support for removing vertices, e.g. del ls[1] removes the second vertex