Add accelerator introspection functionality

[akohlmey]

Summary

This adds introspection functionality for compile time settings of various accelerator packages in LAMMPS.
Included are also added functions for the C-language library interface and the Python module.

Related Issue(s)

This closes #2551
This supersedes and thus closes #2552

Author(s)

Axel Kohlmeyer, Temple U

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).

Backward Compatibility

This is new functionality with no known conflicts.

Implementation Notes

Calling sequence, function names, and behavior differs significantly between C++/C and Python as the different approaches seemed more consistent with existing functionality and function name conventions for each interface. This is not final, so suggestions for changes (with justification) are welcome.

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.