Remove in-house long time interval checking.

[trexfeathers]

🚀 Pull Request

Description

Closes #274. Removes cf-units' own checking for long time intervals that might prevent certain operations.

This was introduced in #72, but cftime (previously netcdftime) continues to improve handling of long time intervals, and has a more informative error when they can't be handled. This makes our own checking superfluous and also overzealous.

I have removed all tests concerning long time interval errors, since it would be inappropriate for our tests to depend on the behaviour of cftime.

If this change is accepted: a sister PR on Iris will also be needed, since Iris includes some defensive coding to avoid failure when printing an invalid time Unit - iris.coords#L346. To allow for cftime's continuing improvements - including tolerance for previously invalid time spacings - it would be safest to use try-except during the printing rather than a more specific check.

Demonstration

my_unit = Unit("months since 2010-01-01", calendar="standard")
print(my_unit.num2date(3))

Traceback (most recent call last):
  File "/tmp/persistent/repos/cf-units/tmp_long.py", line 4, in <module>
    print(my_unit.num2date(3))
  File "/tmp/persistent/repos/cf-units/cf_units/__init__.py", line 2043, in num2date
    return _num2date_to_nearest_second(
  File "/tmp/persistent/repos/cf-units/cf_units/__init__.py", line 564, in _num2date_to_nearest_second
    dates = cftime.num2date(time_values, **num2date_kwargs)
  File "src/cftime/_cftime.pyx", line 499, in cftime._cftime.num2date
  File "src/cftime/_cftime.pyx", line 99, in cftime._cftime._dateparse
ValueError: 'months since' units only allowed for '360_day' calendar

---

my_unit = Unit("months since 2010-01-01", calendar="360_day")
print(my_unit.num2date(3))

2010-04-01 00:00:00

[pp-mo]

Otherwise all looks good

[bjlittle]

@trexfeathers ping (if you get time) 😉

[trexfeathers]

@pp-mo I forgot about this, but it is now ready to merge. It would also fix at least 1 Iris issue. Could you take a quick look? Thanks!

[bjlittle]

@trexfeathers Awesome, thanks! 😄

Just a minor tweak and we're good to go 👍

Review actions have been serviced

[trexfeathers]

Thanks for getting this over the line @bjlittle!