GitLab

- Use bitwise prefix accounting when storing sparse interactions as single pairs in the HIP pair-list kernel. This reduces the number of ballot operations needed to compute per-lane single-pair offsets.
- For HIP CDNA single precision, raise MAX_BITS_FOR_PAIRS to 8 so more sparse interactions are emitted as single pairs instead of full tiles. Keep the existing double precision and RDNA thresholds unchanged.
- Also simplify the HIP LJPME direct correction by computing alpha^2*r2

* Converted more code to common platform

* Converted more code to common platform

* Updated to latest values for physical constants

* Updated documentation on physical constants

* Updated Python unit definitions

* Fixed constants in test case

* Added a comment

All platforms now handle nonbonded exceptions in a consistent way (they ignore cutoffs and periodic boundary conditions).  Also started OpenCL implementation of CustomHbondForce.

Use a consistent (and more accurate) value for electric constant everywhere.  Also cleaned up handling of constants and units in various places.