Add butterworth filtered aperture functions and phase offset correction

[sk1p]

• Both disk and line filter; used by default with hopefully reasonable parameters when creating HoloParams.
• Add phase offset correction, which is needed to properly average complex images
• Tests are mostly smoke tests for now; we will need to develop a strategy how to handle some of the situations (one possibility would be to use pytest-snapshot or similar to make sure things don't break underneath us without noticing)

Contributor Checklist:

• I have added myself to the creators.json file
• I have added a changelog entry for my contribution
• I have added/updated documentation for all user-facing changes
• I have added/updated test cases

[uellue]

Just my 2 ct :-)

[codecov]

Codecov Report

Attention: Patch coverage is 58.22785% with 33 lines in your changes missing coverage. Please review.

Project coverage is 38.04%. Comparing base (1f5d01c) to head (165406b).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
src/libertem_holo/base/reconstr.py	8.33%	33 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #75      +/-   ##
==========================================
- Coverage   39.92%   38.04%   -1.88%     
==========================================
  Files          11       11              
  Lines         541      615      +74     
  Branches       29       42      +13     
==========================================
+ Hits          216      234      +18     
- Misses        314      371      +57     
+ Partials       11       10       -1     

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