Some `GetSamples()` and `GetQOIs()` implementations return `nullptr`

[Martin Modrák]

In both SLMCMC and GreedyMLMCMC the GetSamples() and GetQOIs methods return null. This should be at least mentioned in the documentation, but I don’t really understand why at least for SLMCMC I cannot just get single_chain->GetSamples() - at least when writing to file or computing the means, this is exactly what I get…

[Linus Seelinger]

Thanks for spotting this Martin! I’m now passing samples and QOIs from underlying the single chain through to SLMCMC, so should work as you expect now. For GreedyMLMCMC and MIMCMC where accessing a single chain isn’t well-defined, I added warnings that point users in the right direction for accessing per-level chains.

[Linus Seelinger]

Code has been morged, so should be done! :)