Make bm3d dependency optional

[bwohlberg]

• Make bm3d dependency optional in the sense that only functions and scripts that make direct use thereof cannot be used when it isn't installed
• Some minor packaging improvements

[codecov]

Codecov Report

Merging #243 (442ca69) into main (13be7a3) will increase coverage by 0.01%.
The diff coverage is 72.72%.

@@            Coverage Diff             @@
##             main     #243      +/-   ##
==========================================
+ Coverage   92.62%   92.63%   +0.01%     
==========================================
  Files          49       49              
  Lines        3512     3517       +5     
==========================================
+ Hits         3253     3258       +5     
  Misses        259      259              

Flag	Coverage Δ
unittests	92.63% <72.72%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
scico/linop/abel.py	98.21% <ø> (ø)
scico/ray/tune.py	95.65% <ø> (-0.10%)	⬇️
scico/denoiser.py	91.52% <72.72%> (+0.95%)	⬆️

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