Add xarray

It would be nice to have https://github.com/pydata/xarray on Carnets. I tried to install with `!pip install xarray` but I got the following error (sorry for the funky formatting):

> ERROR: Complete output from command /var/mobile/Containers/Data/Application/B777AD84-D1AE-42AB-9601-23E109F645F5/Library/bin/python3 /var/mobile/Containers/Data/Application/B777AD84-D1AE-42AB-9601-23E109F645F5/Library/lib/python3.7/site-packages/pip/_vendor/pep517/_in_process.py get_requires_for_build_wheel /private/var/mobile/Containers/Data/Application/B777AD84-D1AE-42AB-9601-23E109F645F5/tmp/tmpfus0xv5r:
> ERROR: Traceback (most recent call last):
> File "/var/mobile/Containers/Data/Application/B777AD84-D1AE-42AB-9601-23E109F645F5/Library/lib/python3.7/site-packages/pip/_vendor/pep517/_in_process.py", line 207, in
> main()
> File "/var/mobile/Containers/Data/Application/B777AD84-D1AE-42AB-9601-23E109F645F5/Library/lib/python3.7/site-packages/pip/_vendor/pep517/_in_process.py", line 197, in main
> json_out['return_val'] = hook(**hook_input['kwargs'])
> File "/var/mobile/Containers/Data/Application/B777AD84-D1AE-42AB-9601-23E109F645F5/Library/lib/python3.7/site-packages/pip/_vendor/pep517/_in_process.py", line 48, in get_requires_for_build_wheel
> backend = _build_backend()
> File "/var/mobile/Containers/Data/Application/B777AD84-D1AE-42AB-9601-23E109F645F5/Library/lib/python3.7/site-packages/pip/_vendor/pep517/_in_process.py", line 31, in _build_backend
> ep = os.environ['PEP517_BUILD_BACKEND']
> File "/var/mobile/Containers/Data/Application/B777AD84-D1AE-42AB-9601-23E109F645F5/Library/lib/python3.7/os.py", line 680, in __getitem__
> raise KeyError(key) from None
> KeyError: 'PEP517_BUILD_BACKEND'
> ERROR: Command "/var/mobile/Containers/Data/Application/B777AD84-D1AE-42AB-9601-23E109F645F5/Library/bin/python3 /var/mobile/Containers/Data/Application/B777AD84-D1AE-42AB-9601-23E109F645F5/Library/lib/python3.7/site-packages/pip/_vendor/pep517/_in_process.py get_requires_for_build_wheel /private/var/mobile/Containers/Data/Application/B777AD84-D1AE-42AB-9601-23E109F645F5/tmp/tmpfus0xv5r" failed with error code 1 in /private/var/mobile/Containers/Data/Application/B777AD84-D1AE-42AB-9601-23E109F645F5/tmp/pip-install-tvaed_yf/pandas

Reproduce the `!pip install xarray` command in Carnets and begin with the reported pandas installation failure and its PEP517 `KeyError`. Done means xarray installs successfully in Carnets and can be imported in a notebook.

Written by the indexing model from the issue text.