[FEATURE][API] Add some nice PyQGIS methods and exceptions to QgsLineString

[QGIS-feature-tracker]

Original commit: https://github.com/qgis/QGIS/commit/1e5479964f5b61a908eecb748089d48fc961a422 by nyalldawson

• 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

[DelazJ]

Not sure of what we need to write about, here