Standard duration units

[rantahar]

How should we standardize duration units?

The mHealth total sleep time measure includes a value and a unit. The units can range from picoseconds to years. For total sleep time anything from minute to years could make sense.

Options:

• Require a unit column and handle it in aggregation functions
• Require all rows to use the same unit and record it in a column. Aggregations check that the unit is the same.
• Don't store the unit, assume it's always the same. User handles units.

Maybe I would go with the second. It's relatively simple while still helping the user.

[rantahar]

The mHealth reader should normalize the durations to a single unit for each file read.

[rantahar]

In preprocessing we get duration columns as TimeDelta. These come directly from subtracting DateTime objects. Using them here as well would make sense.

[rantahar]

Solved in PR #300