[Feature Request] Make fix rigid/small compatible with more than 2 billion rigid bodies

[akohlmey]

Summary

Fix rigid/small stores the total number of rigid bodies in a signed 32-bit int and thus limits the number of rigid bodies.

Detailed Description

As supercomputers become more powerful, LAMMPS users are trying to run larger systems. This is (in part) supported by LAMMPS by compiling with -D LAMMPS_BIGBIG. When simulating large molecular systems, the situation gets more complicated. Large amounts of water molecules are often present in such simulations and those often use rigid water models. PR #4037 tries to address one of the issues for simulating such systems with SHAKE constraint, but fix shake cannot always be used.