Sample status/existence

[MJCliffe]

It would be nice to have a flag to say whether the sample still exists or not (would allow for inventory management). Obviously an extension would be sample quantity, but that often isn't known, and what's more important to know is if the sample has been disposed of.

[ml-evs]

This is probably going to be solved by #764, though right now we probably don't handle cases where the status means something like "experiment didn't go as we wanted it to" and "disposed of sample", so maybe we do need two fields.