Update the current processing gain when a new one is uploaded

[stephen-riggs]

Update both the Session and SessionProcessingParameters gain references after a new one is uploaded.

When first done, the SessionProcessingParameters entry will not exist so this must be checked before trying to update.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 34.31%. Comparing base (24d6628) to head (824969a).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #675      +/-   ##
==========================================
- Coverage   34.32%   34.31%   -0.02%     
==========================================
  Files          92       92              
  Lines       11157    11161       +4     
  Branches     1474     1475       +1     
==========================================
  Hits         3830     3830              
- Misses       7202     7206       +4     
  Partials      125      125              

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