Fix simulator test warnings

[fnattino]

To fix #76 :

• use LocalCluster as a context manager to make sure it is properly closed;
• use temporary directories for the workers' scratch directory;

[bkmi]

Before

$ ls
CONTRIBUTING.md  dev  docs  env  LICENSE  notebooks  pyproject.toml  README.md  setup.cfg  setup.py  swyft  swyft.egg-info  swyft-iterator-example.ipynb  tests

run test

$ pytest tests/
=========================================================================================== test session starts ===========================================================================================
platform linux -- Python 3.8.11, pytest-6.2.5, py-1.10.0, pluggy-1.0.0
rootdir: /home/ben/sci/swyft
plugins: anyio-2.2.0
collected 519 items                                                                                                                                                                                       

tests/network_test.py ............................................................................................................................................................................. [ 33%]
...................................................................................                                                                                                                 [ 49%]
tests/prior_test.py .....................sssssssssssssssssssss.............................................                                                                                         [ 66%]
tests/prior_truncator_test.py ......                                                                                                                                                                [ 67%]
tests/simulator_test.py ..................                                                                                                                                                          [ 70%]
tests/store_test.py ..............                                                                                                                                                                  [ 73%]
tests/utils/array_test.py ..........................................................................................................................................                                [100%]

==================================================================================== 498 passed, 21 skipped in 38.84s =====================================================================================

after

$ ls
CONTRIBUTING.md  dask-worker-space  dev  docs  env  LICENSE  notebooks  pyproject.toml  README.md  setup.cfg  setup.py  swyft  swyft.egg-info  swyft-iterator-example.ipynb  tests

unfortunately it's still making the directory.

[fnattino]

There was a last test using Dask among the store tests, it should be now fixed as well!

[bkmi]

looks good to me