Changed docstrings and created TfsFileWriter.open()

Created factory method TfsFileWriter.open():
https://github.com/pylhc/Beta-Beat.src/commit/691f48bbdbd3da2c595c999f71d22ddad74fc4c3#commitcomment-4327337

Added further docstrings:
https://github.com/pylhc/Beta-Beat.src/commit/491367874cf506408939e0ba336951ec8be2c1b5#commitcomment-4327288