add optional automatic time field population to ingest

[tmc]

Having a time field automatically populated with ingestion time if missing would be convenient.

[okayzed]

I'm conflicted on this. On the one hand, timestamps are not required (or are not required to be named "time") but they are generally useful to have in most datasets.

I think adding a flag that says "add timestamp if missing" would be a good compromise, but then most people probably wouldn't realize that the flag exists. OTH, having this be default behavior can lead to confusing situations

Thoughts?