Add Mean Estimator checkpointing #1070
Description
It would be useful if our mean estimator periodically saved off the last mean volume to disk. If the estimate approaches our threshold but fails to converge to the target, the current code will just continue to iterate while killing process will lose the volume.
For larger volume sizes this can often be after a long wait. It is possible the volume is still useful at this point. I'm also not see at this time how the expected target is derived...
While I have not tested, I believe we should be able to use such a checkpoint as a starting point by converting into basis via the adjoint and using as b_coef. As part of this issue we can check this feature.