Implement a pure offline mode

Implement a pure offline mode
The idea is that in the future get_information, get_tan_*, and, for convenience, get_sepa_accounts will be answered from data cached in the FinTSClient object if possible. This information in stored and retrieved with deconstruct()/from_data=
The stay_offline mode allows to inspect this cached data without risking opening a dialogue (and maybe triggering Strong Customer Authentication)