Add ideal gas equation

[svchb]

depends on #599
extracted from #584

[codecov]

Codecov Report

Attention: Patch coverage is 8.57143% with 32 lines in your changes missing coverage. Please review.

Project coverage is 69.03%. Comparing base (eb9f41b) to head (f267d43).

Files with missing lines	Patch %	Lines
...s/fluid/weakly_compressible_sph/state_equations.jl	4.76%	20 Missing ⚠️
src/visualization/write2vtk.jl	0.00%	9 Missing ⚠️
...fluid/weakly_compressible_sph/density_diffusion.jl	0.00%	2 Missing ⚠️
...rc/schemes/fluid/weakly_compressible_sph/system.jl	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #607      +/-   ##
==========================================
- Coverage   69.37%   69.03%   -0.35%     
==========================================
  Files          85       85              
  Lines        5124     5151      +27     
==========================================
+ Hits         3555     3556       +1     
- Misses       1569     1595      +26     

Flag	Coverage Δ
unit	69.03% <8.57%> (-0.35%)	⬇️

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