support chunks in open_groups and open_datatree (#9660)

* support chunking and default values in `open_groups`

* same for `open_datatree`

* use `group_subtrees` instead of `map_over_datasets`

* check that `chunks` on `open_datatree` works

* specify the chunksizes when opening from disk

* check that `open_groups` with chunks works, too

* require dask for `test_open_groups_chunks`

* protect variables from write operations

* copy over `_close` from the backend tree

* copy a lot of the docstring from `open_dataset`

* same for `open_groups`

* reuse `_protect_dataset_variables_inplace`

* final missing `requires_dask`

* typing for the test utils

Co-authored-by: Tom Nicholas <tom@cworthy.org>

* type hints for `_protect_datatree_variables_inplace`

Co-authored-by: Tom Nicholas <tom@cworthy.org>

* type hints for `_protect_dataset_variables_inplace`

* copy over the name of the backend tree

Co-authored-by: Tom Nicholas <TomNicholas@users.noreply.github.com>

* typo

* swap the order of arguments to `assert_identical`

* try explicitly typing `data`

* typo

* use `Hashable` for variable names

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Co-authored-by: Tom Nicholas <tom@cworthy.org>
Co-authored-by: Tom Nicholas <TomNicholas@users.noreply.github.com>