Make tsv_to_excel.R more generic

[berntpopp]

For conversion, the script "tsv_to_excel.R" should accept various separated file formats (including csv).

TODO:

• [x] add argument to explicitly set the separator
• [x] Include logic to determine the separator based on the file extension.
• [x] Include validation logic
• [x] rename script accoringly (maybe convert_to_excel.R)