QCEC 2.1.3 Release: Internal Library Improvements

This release brings some quality of life improvements to the internals of the QCEC library.
First of all, it updates the core/intermediate-representation library (QFR) to its latest version, which is now much more independent from the underlying DD package thanks to cda-tum/mqt-core#244. The core library itself has seen quite some internal updates and code quality fixes.
Secondly, this PR adds better templating to the DD-related functions that allows for better IDE/autocompletion support.
Last but not least, it updates the CMake structure of the project based on cda-tum/dd_package/pull/125 as well as cda-tum/mqt-core/pull/246 to include more warnings, a simpler structure, unified compilation options and much more.

What's Changed

• 🚑 Fixup for newest QFR version by @burgholzer in #221
• 🚸 Better DD package templating by @burgholzer in #222
• ♻️ Refactor CMake Configuration by @burgholzer in #223

Full Changelog: v2.1.2...v2.1.3