Handle period durations of days with T prefix (#86)

* Fixed invalid day offset parsing (closes #77)

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Strengthened tests for `FixInvalidDayOffsetPreprocessor`

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Optimized imports

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Use mockk to mock objects; ICalPreprocessorTest: test stream processor application

* Added ICS test

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Removed ICS test

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Added multiple durations test

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Rename methods and add clarifying comments

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Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>
Co-authored-by: Ricki Hirner <hirner@bitfire.at>
Co-authored-by: Sunik Kupfer <kupfer@bitfire.at>