Fix Small Duration Formatting

[ytaben]

Greetings,

Currently, certain durations that end up with very small nanoseconds end up malformed such that from_str(to_str(duration)) results in an exception.

Specifically:

>>> duration.to_str(timedelta(seconds=0.000492))
'492us5.82077e-11ns'

>>> duration.from_str(duration.to_str(timedelta(seconds=0.000492))
Exception: Unknown unit e in duration 492us5.82077e-11ns

The reason is that we format a very small number with :g which uses scientific notation once it becomes shorter than :f. We also can't use :f because it will results in things like 4.00000s

We get the small number because of

>>> timedelta(seconds=0.000492).total_seconds() * durationpy.duration._second_size
492000.00000000006

I chose rounding nanoseconds because it should be the smallest unit of precision anyway but anything that fixes this is fine.

[icholy]

Thanks for the fix! This is included in 0.7