Add Viscosity Formulation as per Moris et al.

[svchb]

The only other classic formulation that we are missing that is used for Low Reynold Number flows.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 69.66%. Comparing base (708ad9d) to head (4528915).

Files	Patch %	Lines
src/schemes/fluid/viscosity.jl	56.25%	7 Missing :warning:
src/visualization/write2vtk.jl	0.00%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #504      +/-   ##
==========================================
+ Coverage   69.14%   69.66%   +0.52%     
==========================================
  Files          75       75              
  Lines        4582     4586       +4     
==========================================
+ Hits         3168     3195      +27     
+ Misses       1414     1391      -23     

Flag	Coverage Δ
unit	69.66% <50.00%> (+0.52%)	:arrow_up:

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[svchb]

@efaulhaber Please check the equation i think it was wrong in the first version.