fast density

[cpinte]

• Parallelising gas density
• Fixing OpenMP bugs
• Fixing OpenMP gas mass
• Cleaner power-law dust settling
• Fixing typo
• First try at splitting dust density

Type of PR:
Bug fix / new physics / modification to existing code / other

Description:
Give a short description of the change.

Testing:
Describe how you have tested the change

Did you run the botscheck that the code and comments follow the code of conduct? yes/no

Did you update relevant documentation in the docs directory? yes/no

[gemini-code-assist]

Summary of Changes

Hello @cpinte, 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 introduces a significant refactoring of how dust density is managed and calculated throughout the simulation code. The primary change involves renaming the densite_pouss variable to dust_density and clarifying its physical interpretation, often requiring explicit multiplication by nbre_grains to represent total particle counts. This refactoring also includes memory optimizations for scenarios with a single dust grain size and updates to various density calculation, normalization, and radiative transfer routines to align with the new variable semantics. The changes aim to improve the accuracy and efficiency of dust density modeling, particularly in complex astrophysical environments.

Highlights

• Dust Density Variable Refactoring: The variable densite_pouss has been consistently renamed to dust_density across the codebase, clarifying its role and improving code readability.
• Explicit Grain Number Scaling: Calculations involving dust density now explicitly incorporate nbre_grains (number of grains) to correctly represent total particle density, especially when dust_density itself represents a density per grain or a normalized density.
• Memory Optimization for Single Grain Size: The allocation of dust_density in src/mem.f90 has been optimized. When lvariable_dust is false (indicating a single grain size), dust_density is allocated with a dimension of 1 for grain types, reducing memory usage.
• Refined Dust Settling and Normalization: The define_dust_density and normalize_dust_density subroutines in src/density.f90 have been updated to reflect the new dust_density interpretation and to handle normalization more accurately, particularly for variable dust populations and different settling types.
• OpenMP Shared Variable Update: nbre_grains has been added to OpenMP shared clauses in src/SPH2mcfost.f90 and src/dust_prop.f90, ensuring correct parallel execution.

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[gemini-code-assist]

Code Review

This pull request introduces a significant and valuable refactoring by replacing densite_pouss with dust_density. This change effectively separates the spatial distribution of dust from its size distribution, which is a great improvement for code clarity and maintainability. The PR also includes performance enhancements and fixes related to OpenMP.

I've identified one critical bug in src/disk_physics.f90 where an incorrect loop index is used, which will lead to erroneous mass calculations. Additionally, there are some leftover debugging statements in src/density.f90 that should be removed before merging. Overall, this is a solid set of changes once these issues are addressed.

[gemini-code-assist]

These debugging statements and comments should be removed before merging to keep the code clean and avoid unnecessary output.