Use Web Workers for running LAMMPS

[andeplane]

We want to use fix atomify all atomify that will do a callback function to JS whenever a timestep/minimization step is finished so we can synchronize rendering.

But LAMMPS freezes the GUI (naturally), so we want to move the LAMMPS work to another thread. This has (at least) two benefits:

• Faster code (since we actually use two threads)
• naturally solves the GUI hanging problem

See example of web workers. Shouldn't be very hard. Synchronization is performed with post events where we can send a JSON object.

[andeplane]

See https://stackoverflow.com/questions/32291084/minimal-working-example-for-emscripten-webworker