Fix reading of multiple files with ROOTFrameReader

[tmadlener]

BEGINRELEASENOTES

• Fix an issue with reading multiple files via the ROOTFrameReader (#411)
  • Add documentation for API of opening file(s)
  • Add tests for reading multiple files

ENDRELEASENOTES

• Add tests covering this.

[tmadlener]

The check of whether we switched TTrees inside the TChain had to be hoisted a level higher to make it possible to jump back again. The previous implementation implicitly relied on only moving forward. In that case checking that localEntry == 0 was enough to detect TTree changes.