ROOTObjectReader: Add `add_file_extension` to the input file path.

In contrast to the ROOTObjectWriter, the ROOTObjectReader didn't have a add_file_extension for adding .root. This should fix it, plus a check whether the file exists.

Question: Would it be nicer to add a function to the Config class which adds an extension and then checks for the file? One can check in getPath but not after adding the extension...