Streamline Matern parameters

Merged Gordian Edenhofer requested to merge nifty7_break_matern_interface_yet_again into NIFTy_7

Adjust the slope of the Matern kernel such that it resembles the corresponding parameter from the non-parametric amplitude model. Change the docstring accordingly.

Furthermore, amend the docstring of the Matern kernel to state the prior distributions (i.e. the moment-matching) for the various parameters.

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