Warn user when choosing wrong wall viscosity model

[LasNikas]

Allow ArtificialViscosityMonaghan as wall viscosity model but print out warning.
(Otherwise it prints weird cache errors when accidentally choosing the wrong viscosity model

As we can see, it would work...

[codecov]

Codecov Report

Attention: Patch coverage is 37.50000% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 70.57%. Comparing base (6fbb010) to head (d6712db).
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/schemes/boundary/system.jl	0.00%	2 Missing ⚠️
src/schemes/fluid/viscosity.jl	0.00%	2 Missing ⚠️
...chemes/boundary/dummy_particles/dummy_particles.jl	75.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #445   +/-   ##
=======================================
  Coverage   70.57%   70.57%           
=======================================
  Files          66       66           
  Lines        3827     3827           
=======================================
  Hits         2701     2701           
  Misses       1126     1126           

Flag	Coverage Δ
unit	70.57% <37.50%> (ø)

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