Prevent a reference invalidation when wrapped with sample and checkpoint.
Prevents creating a new estimator when loading a sample-wrapped estimator with CheckpointWrapper
(continuation of !82 (merged) which prevented the creation of a new estimator right "below").
This now checks if the estimator is wrapped with SampleWrapper
and updates the estimator at that level.
Fixes bob.bio.gmm#30 (closed).
Edited by Yannick DAYER