bug: interval addition/subtraction should upcast to smallest unit

[jcrist]

What happened?

Currently adding/subtracting intervals results in an interval with a unit of the left operand. This can result in accidental information loss if the smaller unit in the operation is the right operand:

In [1]: import ibis

In [2]: one_second = ibis.interval(seconds=1)

In [3]: two_minutes = ibis.interval(minutes=2)

In [4]: (one_second + two_minutes).type().unit
Out[4]: <IntervalUnit.SECOND: 's'>

In [5]: (two_minutes + one_second).type().unit  # I'd expect seconds here too
Out[5]: <IntervalUnit.MINUTE: 'm'>

In [6]: (one_second + two_minutes).execute()
Out[6]: Timedelta('0 days 00:02:01')

In [7]: (two_minutes + one_second).execute()  # the second is dropped on rounding to minutes
Out[7]: Timedelta('0 days 00:02:00')

The offending code is here. With some pointers I can probably fix this. I know we have casting utilities for handling integer upcasting, but I'm not sure if the same exists for intervals.

What version of ibis are you using?

dev

What backend(s) are you using, if any?

No response

Relevant log output

No response

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