Add recursion to Configuration File parsing

[drodarie]

When a file is parsed into a Configuration dictionary through a ReferenceParser and has a reference to an external file, this file has to be "parsable" with the same ReferenceParser. Also that external file is only converted to a dictionary so any reference within that file will not be parsed. Finally, it might be worth to merge ConfigurationParser and ReferenceParser

[drodarie]

I have currently two questions:

• To implement the tests for this feature, I would need to register a mock ConfigurationParser declared in the test file that could be found by get_configuration_parser_classes but I do not know how to do that.
• Regarding the paths to referenced files in configuration files, is it better that they work from the root of the script or from the perspective of the files. Especially, when we consider re-usability of configuration files, using path relative to the files might simplify the combination of configuration files from the user perspective?