Make bm3d and bm4d optional

[bwohlberg]

Move bm3d and bm4d to examples requirements.

[codecov]

Codecov Report

Merging #458 (b068dcb) into main (216ffc8) will decrease coverage by 0.40%.
The diff coverage is n/a.

❗ Current head b068dcb differs from pull request most recent head 6320757. Consider uploading reports for the commit 6320757 to get more accurate results

@@            Coverage Diff             @@
##             main     #458      +/-   ##
==========================================
- Coverage   93.34%   92.94%   -0.40%     
==========================================
  Files          87       87              
  Lines        5512     5512              
==========================================
- Hits         5145     5123      -22     
- Misses        367      389      +22     

Flag	Coverage Δ
unittests	92.94% <ø> (-0.40%)	⬇️

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see 2 files with indirect coverage changes