Support PAR_ONLY model

[hyschive]

Description

In our current temporary approach for particle-only simulations, we enable OPT__FREEZE_FLUID and assign a negligibly small density to gas/FDM to effectively ignore their contribution to gravity. However, this method is inefficient as it is memory-intensive (requiring allocation of all gas/FDM variables), storage-heavy (since all gas/FDM variables are still saved to disk), and time-consuming (because even with OPT__FREEZE_FLUID, gas/FDM is still evolved with a zero time-step). This feature request seeks to address these inefficiencies by implementing proper support for genuine particle-only simulations.

Tasks

• Only need to allocate the DENS field in the fluid array
• No need to evolve the fluid array
• Update the gamer frontend in yt

Related PRs

• Optimize OPT__FREEZE_FLUID and OPT__FREEZE_PAR #355
• Generalize fluid[] #135