1580 Reuse flow difference vector

[HenrZu]

Changes and Information

Please briefly list the changes (main added features, changed items, or corrected bugs) made:

• reusable flow-difference vector is allocated once per simulation
• and used for both right-hand-side evaluations and compartment reconstruction

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.47%. Comparing base (52329fe) to head (15660ed).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1581      +/-   ##
==========================================
- Coverage   97.48%   97.47%   -0.02%     
==========================================
  Files         190      190              
  Lines       16030    16036       +6     
==========================================
+ Hits        15627    15631       +4     
- Misses        403      405       +2     

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[reneSchm]

I am a little surprised that this used to allocate at all. I assumed this would be an Eigen difference expression. Do you know where exactly the allocation occurs?

[reneSchm]

I don't think exposing a temporary is a good idea, maybe make it protected? Or just use one vector each, the storage cost is not too bad.