Write xarray data back to xarray

[stefsmeets]

This PR adds a way to sync the dataset back to the hdf5 file. This updates in-place.

from imas2xarray import to_imas, to_xarray

variables = ('rho_tor_norm', 'time', 't_e')
ids = 'core_profiles'

dataset = to_xarray('./data', ids=ids, variables=variables)

print(dataset['t_e'])
dataset['t_e'] += 1

to_imas('./data', dataset=dataset, ids=ids, variables=variables)

Closes #2

Todo

• Add tests
• Tweak scripts/modify_data.py and add example to docs
• Wrap .open_ids in contextmanager
• Update docstrings
• Check that variables are in var_lookup or handle this otherwise
• Fix tests in CI