Inline viscosity

[efaulhaber]

Okay, hear me out. This is a 1.4-1.5x speedup in the fluid-fluid interact, just by inlining dv_viscosity. Don't ask me why the difference is so extreme.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

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Report is 1 commits behind head on main.

Files	Patch %	Lines
src/schemes/fluid/viscosity.jl	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #578   +/-   ##
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  Coverage        ?   69.73%           
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  Files           ?       76           
  Lines           ?     4573           
  Branches        ?        0           
=======================================
  Hits            ?     3189           
  Misses          ?     1384           
  Partials        ?        0           

Flag	Coverage Δ
unit	69.73% <66.66%> (?)

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