Provide a fake bank with a Weboob backend

Follow up of #619 (closed), and can be tackled as soon as !430 (merged) is merged.

!430 (merged) will allow to overload the sources.list file in use by Weboob. We could add a file://path_to_kresus/test_data/weboob line in the sources.list file to look for modules in both the default Weboob repository as well as test_data/weboob.

This way, we could write a sample Weboob module providing a CapBank interface which could be used as a Fake Bank with a Weboob backend, which would be super interesting to test and debug Weboob interface.