Support intervalds and/or intervaldsus

**Is your feature request related to a problem? Please describe.**
It would be convenient to store time durations directly in the DB rather than storing it as a number of seconds.

**Describe the solution you'd like**
Use a Python timedelta to map to the Comdb2 intervalds or intervaldsus values. This seems like a natural mapping.

**Describe alternatives you've considered**
The alternative is to use a double or an int (depending on the needed precision) to store the duration as a number of seconds.