Tracer particles for FDM

[hyschive]

Goals

Support tracer particles for FDM

Tasks

• Compute FDM bulk velocity in Par_UpdateTracerParticle.cpp
• How do we handle density void (e.g., vortices) that leads to extremely high local velocity?

Optional tasks

• Add FDM bulk and thermal velocity as built-in derived fields in Flu_DerivedField_BuiltIn.cpp
• Support both built-in and user-defined derived fields in Prepare_PatchData.cpp

Notes

• This can be useful for FDM cosmological zoom-in simulations by refining regions occupied by tracer particles.