Artificial Density Diffusion should only be applied to the same system

[svchb]

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[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 69.90%. Comparing base (05a11e9) to head (fa5f166).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...fluid/weakly_compressible_sph/density_diffusion.jl	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #608   +/-   ##
=======================================
  Coverage   69.89%   69.90%           
=======================================
  Files          86       86           
  Lines        5122     5123    +1     
=======================================
+ Hits         3580     3581    +1     
  Misses       1542     1542           

Flag	Coverage Δ
unit	69.90% <75.00%> (+<0.01%)	⬆️

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