More generic posterior sampling

It was mentioned that the machinery for drawing posterior samples should be made more general, and should not only be available for WienerFilterCurvature.

I'd be happy to implement this, but I need a clear description of the required ingredients and the algorithm.

Concerning this functionality, there are currently several open questions:

• issue #211 (closed)
• issue #200 (closed)
• NIFTy4 currently has two implementations: generate_posterior_sample() and generate_posterior_sample2(), both in nifty4/library/wiener_filter_curvature.py. Which is the correct one?

@kjako, @reimar, @parras: please let's get together and sort this out soon!