Store time units in BDV images

[stefanhahmann]

For some analysis of the lineage graph, it would be nice, if BDV images could store the time units.
At the moment, it is possible to use the timepoints as a proxy, but this relies on the assumption that time deltas between all the images are roughly the same, which may not be the case in every possible scenario.

Cf.

• Add further features for spots and branch graph mastodon-sc/mastodon-deep-lineage#1
• Generation time of a BranchSpot mastodon-sc/mastodon-deep-lineage#7