Implement indexer like ``iloc`` for ``TextFileReader`` objects.

[ludwiglierhammer]

Problem

In #242 we had a disscussion how to implement indexer like iloc for TextFileReader objects (see #242 (review)).

Since we couldn't solve this issue in that PR it is reasonable to create this new issue which may lead to a new PR.

Tasks

• implement indexer like iloc for TextFileReader objects
• restructure and simplify magic method __getattr__ for DataBundles