Quantum-Dynamics-Hub/libra-code: Libra with the ANN; new xTB/CP2k workflows; generalized and simplified workflows; libint2 and pickling support;

Major additions and changes:

• ANN functionality is revised and extended, now fully-functional
• Eigen3 is removed from the codebase, build instructions are now set to use the external package
• Added the libint2 support, Libra is linked to the external libint2 libraries and can use them
• Added the MOLDEN-file procedures: constructing the molecular integrals using these files
• Added the CP2k workflow with libint2 support
• Added the xTB (in CP2k) workflow
• Pickling support for various Libra classes
• Simplified and generalized the multithreading calculations and made the workflows more user-friendly
• Added more efficient methods for SD overlap and time-overlap calculations
• Added user-friendly general NA-MD workflows, with a set of new recipes for common types of calculations
• Less memory-demanding saving in the files
• Fixed a memory leak in the NA-MD workflows
• Moved some examples to the CyberTraining repository