Do not hold Python's GIL when running commands

[Luthaf]

Description

Fixes #1128. I can confirm this fixes the issue for metatensor actions.

However, I'm not sure how to test that pycv still works fine, is it included in the CI test suite?

Type of contribution

• changes to code or doc authored by PLUMED developers, or additions of code in the core or within the default modules
• changes to a module not authored by you
• new module contribution or edit of a module authored by you

Copyright

• I agree to transfer the copyright of the code I have written to the PLUMED developers or to the author of the code I am modifying.

• the module I added or modified contains a COPYRIGHT file with the correct license information. Code should be released under an open source license. I also used the command cd src && ./header.sh mymodulename in order to make sure the headers of the module are correct.

Tests

• I added a new regtest or modified an existing regtest to validate my changes.
• I verified that all regtests are passed successfully on GitHub Actions.

[Iximiel]

However, I'm not sure how to test that pycv still works fine, is it included in the CI test suite?

pycv has its workflow run in linux(-pycv-mpi-), which runs its regtests in its plugin subdirectory.

But we do not have a "run pycv from plumed in python" test that runs on the CI

[GiovanniBussi]

However, I'm not sure how to test that pycv still works fine, is it included in the CI test suite?

pycv has its workflow run in linux(-pycv-mpi-), which runs its regtests in its plugin subdirectory.

But we do not have a "run pycv from plumed in python" test that runs on the CI

This possibly means that, due to these GIL issues, also "run pycv from plumed in python" is not working? I though @Iximiel was testing this at least locally, is this correct? Otherwise I think we need a test

[Iximiel]

This possibly means that, due to these GIL issues, also "run pycv from plumed in python" is not working? I though @Iximiel was testing this at least locally, is this correct? Otherwise I think we need a test

I tried, and I think I need to find a solution to this:

what():  Can't add new modules after the interpreter has been initialized

[Luthaf]

Is this an error on the current master or with these changes?

[Iximiel]

Is this an error on the current master or with these changes?

The error is in master, don't worry.

I opened a separate issue about it