(Sinktree) Wrong fallback behaviour when sink are not in cache

[Yrisch]

Description:
I discovered a wrong behaviour of the sinktree method while debugging #699. If we had a cache miss, the fallback was on the wrong array (if the neighbour was a sink). Difficult to detect that with tests, as the number of parts is set to 1000 (same as the cache size) in most cases.

Also removed the ptmass killing in 'accrete_particles_outside_sphere' as it had unexpected behaviour with the subgrouping module...

Components modified:

• Setup (src/setup)
• Main code (src/main)
• Moddump utilities (src/utils/moddump)
• Analysis utilities (src/utils/analysis)
• Documentation (docs/)

Type of change:

• Bug fix
• Physics improvements
• Better initial conditions
• Performance improvements
• Documentation update
• Other (please describe)

Did you run the bots? no

Did you update relevant documentation in the docs directory? no

Did you add comments such that the purpose of the code is understandable? no

Is there a unit test that could be added for this feature/bug? yes

If so, please describe what a unit test might check:
We should increase the number of parts in testgrav

[gemini-code-assist]

Summary of Changes

Hello @Yrisch, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical bug in the sinktree method where the system was incorrectly handling cache misses for sink particle data, leading to the retrieval of information from the wrong memory arrays. My changes ensure that when sink particle data is not in the cache, the system correctly falls back to the dedicated sink particle data structures for accurate force computations. Additionally, I've refined the particle accretion logic to prevent unintended processing of sink particles in a general accretion routine.

Highlights

• Bug Fix: Sink Particle Data Retrieval: I've corrected the compute_forces subroutine in src/main/force.F90. Previously, when data for a sink particle was not found in the cache, the system would incorrectly fall back to retrieving its coordinates (lines 1269-1277) and smoothing length (lines 1289-1299) from the general xyzh array. This change ensures that if the neighbor is identified as a sink particle (iamsinkj), its data is correctly retrieved from the xyzmh_ptmass array, preventing erroneous force calculations.
• Sink Particle Accretion Logic Refinement: In src/main/part.F90, I've removed a redundant or incorrect loop within the accrete_particles_outside_sphere subroutine (lines 2111-2117). This loop was processing xyzmh_ptmass (which holds sink particle data) for accretion. The change ensures that this specific accretion routine now only applies to regular particles stored in xyzh, aligning with the expected behavior for sink particles which may have different accretion mechanisms.

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[gemini-code-assist]

Code Review

This pull request fixes an important bug in the sinktree method where a cache miss for a sink particle would lead to using an incorrect data array. The changes in force.F90 correctly address this by reordering the logic to check the cache first, and then fall back to the appropriate array for sink or SPH particles. This looks good.

I have one major point of feedback regarding a change in part.F90. A loop that deletes sink particles outside a certain radius has been removed. This is a significant functional change that is not mentioned in the pull request description. I've requested clarification to ensure this is intended and to suggest documenting it.

I also left a minor style comment in force.F90 to improve code consistency.