Feat/configparser update 18

Please review the feature update.

The update allows to specifiy 'required' properties in the validator as requested from Andrey @s9150874--tu-dresden.de. A completeness check is conducted after reading all properties and will result in an IllegalStateException if not all required properties are set.

I also used the opportunity to simplify the usage of the parser. Before it was necessary to do a '2-stage' parsing, where you first parse the default config file, extract the default printer and format configs and pluck them into a new constructor for the actual parser. The constructor now simply takes two files and both files are parsed with the same parser instance.

Javadoc is extended and tests are updated and passed.