Allow non-constant imputation

[gbowlin]

          Dumb question, would we ever want to impute a different value because of the timestamp? 

Like if no value is recorded for this event, use the prior value?

I'm not sure what use cases we have right now for impute other than a flag to say "the event happened"

Originally posted by @gbowlin in https://github.com/epic-open-source/seismometer/pull/76#discussion_r1745968993