Neutral physics for Fci

[totork]

Purpose

This PR adjust the neutral_mixed model to run in Fci.

Change Summary

• Setting the BCs differently when running in Fci
• Using a different perp. diffusion operator in Fci
• Disable setting certain BCs when running MMS tests
• Use Field3DParallel for parallel derivatives

Validation

One 2D MMS test that tests density and pressure diffusion and conduction. More tests soon (1D).

AI Assistance

None

Documentation

Some comments in the code

Review Notes

Generally, the code seems quite consistent. At some places, we apply BCs after calculating quantities from fields that we also set BCs for, which might introduce inconsistencies. Something like
logPnlim = log(Pnlim); logPnlim.applyBoundary();
This might make sense physics-wise, but obviously kills MMS tests as these destroy the error convergence. @mikekryjak Is there a reason why we e.g. set these BCs for certain variables specifically?

[codecov]

Codecov Report

❌ Patch coverage is 59.04762% with 43 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.57%. Comparing base (7b61dd8) to head (4dd64ae).

Files with missing lines	Patch %	Lines
src/neutral_mixed.cxx	59.04%	21 Missing and 22 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #665      +/-   ##
==========================================
- Coverage   59.75%   59.57%   -0.18%     
==========================================
  Files          98       98              
  Lines       10274    10311      +37     
  Branches     1482     1499      +17     
==========================================
+ Hits         6139     6143       +4     
- Misses       3488     3507      +19     
- Partials      647      661      +14     

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