Revert "changed sample to std::vector<sample> in mcmc_writer"

This reverts commit bafa4b48d1515918994dd4d283eda3886d42a99a.
Revert "replaced sample with std::vector<sample> so ensembles can be added"

This reverts commit 4370fe60a35b6b5e0ce27890ecca897cf8bcca98.