Use 64bit integers to store prev timestep

[jhenin]

Should fix #639

[giacomofiorin]

Looks good to me!

For reference:

• NAMD was not affected by the bug because size_t is already 64-bits (only unsigned).
• LAMMPS was affected, although it's unclear how frequently: jobs of > 2^32 steps are very rare
• patched GROMACS versions were affected.

What about GROMACS 2024?

[jhenin]

In GROMACS 2024, Colvars gets the current time step explicitly through the ForceProvider API, so not affected. https://github.com/Colvars/colvars/blob/7934bb674bfe071f3119e0430811ca638c5209b5/gromacs/gromacs-mdmodules/applied_forces/colvars/colvarsforceprovider.cpp#L318

[jhenin]

Does the standard specify that size_t is 64 bit? Maybe in practice only? https://en.cppreference.com/w/c/types/size_t

[giacomofiorin]

Does the standard specify that size_t is 64 bit? Maybe in practice only? https://en.cppreference.com/w/c/types/size_t

I believe you'd have to go to a 32-bit architecture to find a narrower type for size_t.