Reduce memory usage of noise optimisation

[ggmarshall]

• Remove fft field when not needed
• Make the fft plot process in batches to reduce memory - need lgdo update to do this

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[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 56.22%. Comparing base (83bd5cb) to head (ac6fc19).

Files with missing lines	Patch %	Lines
src/pygama/pargen/noise_optimization.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #686      +/-   ##
==========================================
+ Coverage   55.20%   56.22%   +1.02%     
==========================================
  Files          61       61              
  Lines        8354     8357       +3     
==========================================
+ Hits         4612     4699      +87     
+ Misses       3742     3658      -84     

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