Add ability to send positions to CP2K via file

Introduces the ability to side-step issues in passing large atomic structures to CP2K through stdin for MPI applications. Adds a set_pos_file option that sends positions via file rather than stdin, which is a route others have used successfully and one I've confirmed fixes issues with ASE.

My PR to CP2K which supports the new route has been merged, but we'd need to test against the main branch until the next CP2K release occurs.

Checklist

• I am familiar with ASE's contribution guidelines.
• Doc strings in code changed in this MR are up to date.
• Unit tests have been added for new or changed code.
• Issue is resolved via "closes #XXXX" if applicable.
• Add warning about using the "set_pos_file" for structures larger than a few dozen atoms
• Check the shell version so we can warn users if their CP2K is out of date.