Add hybrid modeling with neural-network-based binding models

[jbreue16]

This PR introduces hybrid models to CADET-Core, allowing the binding model to be replaced or approximated by a pre-trained neural network.

Todo

• rebase code
  • resolve conflicts
  • add changes to generalized unit operations
  • squash commits, are there meaningful separate commits?
• fix mkl inclusion (src/mkl_files)
• add documentation
• add more cadet-core tests?
• add simple python code example to cadet-verification, where a NN is trained and plugged in

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[schmoelder]

Unfortunately, rebasing was not possible without additional work. Amongst other issues, the PR includes changes to the unit operation models and it's not quite clear yet as to why. I see the following options:

• Investigate what other changes have been made in this PR. Maybe we're lucky and we figure it out quickly, but it could also be tedious
• Start from scratch, using this PR merely as inspiration

Any preference @lieres?

[lieres]

@jazib-hassan-juelich, please have a look and comment.