Score-based density estimators for SBI

[jsvetter]

What does this implement/fix? Explain your changes

This PR implements score-based methods for SBI (and related methods like flow matching). The first goal is to have a running version for score-based NPE. Later-on NLE, sequential methods and methods that can deal with multiple observations can be tackled.

This includes new base-classes for the required vectorfield-estimators and posteriors.

Does this close any currently open issues?

Fixes #962

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[manuelgloeckler]

Related to #963 and #966 .

[janfb]

@pfuhr do you agree to your contributions being re-licensed under Apache 2.0 in the future?

[pfuhr]

Yes, I agree that my contributions are re-licensed under Apache 2.0 in the future.