DataConverter.merge_files loops over letters in string when filepath is given as string

[lhennig]

Normally, DataConverter.merge_files should be called with a list of strings. If I (accidentally) only supply one filepath as a string, it will not raise an exception, but instead will loop over the letters in the string and create empty files having the letter as a name. E.g., the code

input_dir = "data"
converter.merge_files(files=input_dir)

will create empty files "d", "a", and "t" in the current working directory. It would be better if an exception is raised.