QUARK 2.1.1

QML Architecture Update (by @GreshmaShaji and Florian Kiwit): Architecture restructuring for future QML applications. Introduced new abstract base classes (DataHandler, Model, Circuit, Training) to ensure scalability and modularity.

Implement PEP8-Standard Code Style (by @GreshmaShaji and @Marvmann): In order to improve readibility and harmonize the style of the code, we implement changes towards the PEP 8 coding guideline.

Full changelog: v2.1...v2.1.1