Update to metatensor-torch v0.5.5

[Luthaf]

Description

The main change is that there is now an official "features" output that we can use to pass CV, instead of the custom "plumed::cv" output. The code still accepts models using the previous names, but outputs a deprecation warning.

This PR also adds a link to a page in metatensor documentation that is not yet online, but will be as soon as metatensor/metatensor#726 is merged.

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.

[Luthaf]

Thanks @GiovanniBussi!