Enable callback to JS during a run and minimize command

[andeplane]

If you have a LAMMPS script with a run 1000 command or a minimization command, the caller of LAMMPS.js should be able to get callbacks every N timesteps to show values of computes, variables and fixes in addition to real time updates of atomic positions and other relevant values. See https://github.com/andeplane/atomify/blob/main/cpp/lammpsweb/fix_atomify.cpp as an example implementation (requires some patches like seen in https://github.com/andeplane/atomify/blob/main/cpp/lammps.patch#L62).

In general:

• we use a Fix to get a callback mechanism into LAMMPS
• it is invoked on our choice of steps during a timestep (e.g. END_OF_STEP)
• there we can call a callback function defined in JS like this: https://github.com/andeplane/atomify/blob/main/cpp/lammpsweb/fix_atomify.cpp#L165