Refac all the viscous riemann solves

[sbryngelson]

idk why this is another branch but it matches up with #846

[codecov]

Codecov Report

Attention: Patch coverage is 38.04878% with 127 lines in your changes missing coverage. Please review.

Project coverage is 43.23%. Comparing base (ba33af4) to head (0db9b67).

Files with missing lines	Patch %	Lines
src/simulation/m_riemann_solvers.fpp	38.04%	106 Missing and 21 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #847      +/-   ##
==========================================
- Coverage   43.24%   43.23%   -0.01%     
==========================================
  Files          68       68              
  Lines       20183    19985     -198     
  Branches     2402     2392      -10     
==========================================
- Hits         8728     8641      -87     
+ Misses       9986     9885     -101     
+ Partials     1469     1459      -10     

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[sbryngelson]

Closed in favor of #846