Switch the multiprocessing default method to spawn

[cdiener]

Checklist

• There are no similar issues or pull requests for this yet.
• The request is not specific for MICOM Qiime 2 plugin (q2-micom)

Is your feature related to a problem? Please describe it.

There are repeated problems with processes not returning, excessive memory usage, and
Jupyter weirdness.

Describe the solution you would like.

We should test switching to spawn for all workflows. Hopefully, the overhead is not too bad.

Describe alternatives you considered

Forkserver did not perform much better. Deactivating parallel computing would not be popular.

Additional context

None.