Io fix

[christophwelling]

Fixes 2 issues related to reading eventfiles:

• Stops NuRadioRecoio from crashing because self.__buffer_size is not set if files are scanned
• The latest tinydb update renamed the Table.purge function to Table.truncate. https://tinydb.readthedocs.io/en/latest/changelog.html?highlight=purge#v4-0-0-2020-05-02 This also means, that we now require a newer tinydb version, which has been added to the pyproject and the documentation