Failure when creating 'T: mean within days time: minimum over days'' collapse

[davidhassell]

At v3.16.2, the following fails in the following way, when it shouldn't:

>>> import cf
>>> f = cf.example_field(5)
>>> f.collapse("T: mean within days time: minimum over days", within_days=cf.h(12))
Traceback (most recent call last):
    ...
TypeError: unsupported operand type(s) for %: 'TimeDuration' and 'TimeDuration'

Climatological time collapses "within/over years" seem OK, but here are no units tests for a "within/over days" case!