Enable valdation of smaller extractions

[tsodring]

Hi,

Following up on #47, when debugging the code, it seems the validator assumes the extraction contains loependeJournal, endringslogg and offentligjournal. Some extractions may not contain these files. An improvement suggestion is that the tool knows what the minimum set of files contained in an extraction are, and the tool is able to validate smaller specialised system (fagsystem) extractions.