User defined right hand side function

[agdestein]

This PR has some breaking changes.
Now the user can choose the force (add body force, closure terms etc.).
By default, it is just decaying turbulence (convection + diffusion).

Breaking changes

• Setup now expects a viscosity visc instead of Reynolds number Re.
  Do Setup(; ..., visc = 1e-3) instead of Setup(; ..., Re = 1e3).
  This is more transparent, as the Reynolds number cannot be prescribed when u is changing during the simulation, but the viscosity is constant.
• The right hand side is now flexible. In solve_unsteady, it defaults to navierstokes! without temperature equation, and boussinesq! with temperature equation. This can be overridden by doing solve_unsteady(; force! = custom_force!, ...).
• solve_unsteady should now be solve_unsteady(; start, ...) instead of solve_unsteady(; ustart, tempstart, ...), where start is either start = (; u) or start = (; u, temp). The force! acts on a state state = (; u) or state = (; u, temp), and writes the forces to a second state containing the forces for u and temp respectively.
• Body forces and closure models are applied in a user-provided force! function to the solver, and not hard-coded in the Setup function. See the examples folder for examples.

Other changes

A new fast spectral-like Poisson solver psolver_transform is available, that uses DCT in addition to FFT. This allows for periodic directions and walls.

[codecov]

Codecov Report

Attention: Patch coverage is 75.69989% with 217 lines in your changes missing coverage. Please review.

Project coverage is 62.79%. Comparing base (a9ecfb0) to head (0b303f6).
Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
src/eddyviscosity.jl	0.00%	82 Missing ⚠️
src/operators.jl	82.88%	57 Missing ⚠️
src/solver.jl	53.22%	29 Missing ⚠️
src/pressure.jl	82.35%	12 Missing ⚠️
src/matrices.jl	86.84%	10 Missing ⚠️
src/initializers.jl	55.55%	8 Missing ⚠️
src/processors.jl	66.66%	7 Missing ⚠️
src/precompile.jl	0.00%	3 Missing ⚠️
ext/MakieExt.jl	50.00%	2 Missing ⚠️
src/utils.jl	97.36%	2 Missing ⚠️
... and 5 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #150      +/-   ##
==========================================
+ Coverage   59.08%   62.79%   +3.70%     
==========================================
  Files          26       22       -4     
  Lines        3556     2857     -699     
==========================================
- Hits         2101     1794     -307     
+ Misses       1455     1063     -392     

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