Draft: Gauss markov processes

Open Vincent Eberle requested to merge gauss_markov_processes into NIFTy_8

@pfrank I also added the IWPProcess from Resolve and added P.Arras as author.

The interface is not the same, but I tried to extend it a bit to be able to use it for the spectral models. If you have time for a short review, that would be great.

PS: There is no rush to merge this soon.

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