Add chunks='auto' support for cftime datasets

[charles-turner-1]

• Closes #xxxx
• Tests added

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[jemmajeffree]

Would these changes also work for cf timedeltas or are they going to still cause problems?
I'm tempted to write a script to bash through all the ACCESS-NRI intake datastores and see if there's anything else in there that's dtype object — let me know if this would be useful, or if we should just wait for it to break later

[charles-turner-1]

Would these changes also work for cf timedeltas or are they going to still cause problems? I'm tempted to write a script to bash through all the ACCESS-NRI intake datastores and see if there's anything else in there that's dtype object — let me know if this would be useful, or if we should just wait for it to break later

If you can find something thats specifically a cftimedelta and run the _contains_cftime_datetimes function on it that'd be super helpful to know whether it returns True or False.

[jemmajeffree]

TLDR: don't mind me, it's not going to cause any issues

Firstly, what I thought was a cftimedelta turned out to be a numpy timedelta hanging out with a cftime

When I did manage to coerce this timedelta into cftime conventions, it just contained a floating point number of days, so I can't see anything having issues with its size

coder = xr.coding.times.CFTimedeltaCoder()
result = coder.encode(oops.average_DT).load()
print(result.dtype)
result

[charles-turner-1]

I did some prodding around yesterday and I realised this won't let us do something like

import xarray as xr
cftime_datafile = "/path/to/file.nc"
xr.open_dataset(cftime_datafile, chunks='auto')

yet, only stuff along the lines of

import xarray as xr
cftime_datafile = "/path/to/file.nc"
ds = xr.open_dataset(cftime_datafile, chunks=-1)
ds = ds.chunk('auto')

I think implementing the former is going to be a bit harder, but I'm starting to clock the code structure a bit more now so I'll have a decent crack.