Check ValidationUtils for negative Susceptance

[johanneshiry]

At the moment it is not clear if a negative transformer susceptance is accepted and marked as valid within the ValidationUtils. However, transformer susceptance values are always negative while positive ones are not possible.

In order to ensure that negative values are allowed the ValidationUtils needs to be checked and maybe adapted in order to allow only negative values for the transformer susceptance.

Furthermore, all test data should be adapted accordingly to have only negative susceptance values in order to avoid confusion.