Add ideal gas equation

[svchb]

depends on #599
extracted from #584

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 68.54%. Comparing base (1d007c0) to head (1cab8eb).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/visualization/write2vtk.jl	0.00%	4 Missing ⚠️
...s/fluid/weakly_compressible_sph/state_equations.jl	92.30%	1 Missing ⚠️
...es/fluid/weakly_compressible_sph/state_equation.jl	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #607      +/-   ##
==========================================
+ Coverage   68.47%   68.54%   +0.07%     
==========================================
  Files          87       87              
  Lines        5360     5389      +29     
==========================================
+ Hits         3670     3694      +24     
- Misses       1690     1695       +5     

Flag	Coverage Δ
unit	68.54% <80.00%> (+0.07%)	⬆️

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[efaulhaber]

Blocked by #669.