netcdftime.num2date() rounding issues

[djkirkham]

When unpinning Matplotlib it was discovered that there are rounding issues with netcdftime.num2date() now that it implements microsecond precision. This issue likely affects many other areas of the code. For instance, coordinate categorisation: #2137

[marqh]

I think this issue has now been addressed. @djkirkham please may you check and close this issue or update it with more details, based on what you find

thank you
mark

[djkirkham]

It's not fixed in the iris.unit module, which is deprecated. Does anything need to be done about that?

[djkirkham]

iris.unit has been removed.