Implement Koopman-based Spectral Decomposition

[nicrie]

Is your feature request related to a problem? Please describe.
Spectral decomposition is currently limited to linear Hilbert based approaches which sometimes miss key features when dealing with more complex and non-linear data.

Describe the solution you'd like
Implement the operator-theoretic spectral decomposition method from this paper. I haven't understood the details yet, but from what I get they build an approximation of the Koopman operator to capture the dynamics of complex systems.

Describe alternatives you've considered
None.

Additional context
None.