Add missing functionality in TransformedRangeView

Summary

Add the methods operator[] and empty() to the TransformedRangeView.

Details

The TransformedRangeView is a utility to transform the values on access by a given function. This access is implemented via iterators and - in a strict sense - a range is just a pair of iterators. But it is very convenient to provide also direct element access functions via operator[] in case the wrapped range has such an access. The function can be applied on that direct access as well.