added UnitNotDetectedException
implemented UnitNotDetectedException in WeighUnit and UT
implemented UnitNotDetectedException to LengthUnit & unittest
removed ToDos from LengthUnit and WeightUnit
fixed errormessages and added input validation to conversions
reworked code formatting in WeightUnit
fixed errormessages in LengthUnit
removed finished ToDo from LengthUnit
removed finished ToDo from WeightUnit
implemented UnitNotDetectedException in SiUnitFactory
implemented UnitNotDetectedException in SiUnit
updated unit tests for SiUnit for new Exception