Exit cleanly when i-PI drops the connections rather than erroring out.

[ceriottm]

This prints the final stats, and is in general cleaner.

Summary

fix_ipi exits with an error when the server disconnects. this PR implements a cleaner exit mechanism, allowing among other things to print out the runtime stats that LAMMPS usually prints at the end of a simulation.

Author(s)

Michele Ceriotti, EPFL, michele.ceriotti@gmail.com

Licensing

By submitting this pull request, I agree, that my contribution will be included in LAMMPS and redistributed under either the GNU General Public License version 2 (GPL v2) or the GNU Lesser General Public License version 2.1 (LGPL v2.1).

Artificial Intelligence (AI) Tools Usage

By submitting this pull request, I confirm that I did NOT use any AI tools to generate
all or parts of the code and modifications in this pull request.

Backward Compatibility

This should be fully backward compatible.

Implementation Notes

This simply sets a flag when the socket drops the connection, which is picked up at the end of the verlet steps to trigger the usual LAMMPS exit path.

Post Submission Checklist

• The feature or features in this pull request is complete
• Licensing information is complete
• Corresponding author information is complete
• The source code follows the LAMMPS formatting guidelines
• Suitable new documentation files and/or updates to the existing docs are included
• The added/updated documentation is integrated and tested with the documentation build system
• The feature has been verified to work with the conventional build system
• The feature has been verified to work with the CMake based build system
• Suitable tests have been added to the unittest tree.
• A package specific README file has been included or updated
• One or more example input decks are included

[Luthaf]

Shouldn't this be sent upstream instead of here?