529 Make pynwb optional dependency

[dfsp-spirit]

Changes Summary

• see pynwb not available for powerlinux on conda-forge #529 for the motivation

Reviewer Checklist

• Are testing routines present?
• Do objects in the global package namespace perform proper parsing of their input?
• Are all docstrings complete and accurate?
• Is the CHANGELOG.md up to date?

[codecov]

Codecov Report

Patch coverage: 50.00% and project coverage change: +0.04 🎉

Comparison is base (0ad96b6) 68.99% compared to head (53f55c4) 69.03%.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #530      +/-   ##
==========================================
+ Coverage   68.99%   69.03%   +0.04%     
==========================================
  Files          87       87              
  Lines       10081    10103      +22     
  Branches     2112     2120       +8     
==========================================
+ Hits         6955     6975      +20     
+ Misses       2550     2544       -6     
- Partials      576      584       +8     

Impacted Files	Coverage Δ
syncopy/datatype/continuous_data.py	79.11% <37.50%> (-0.94%)	⬇️
syncopy/datatype/discrete_data.py	80.86% <40.00%> (-0.70%)	⬇️
syncopy/io/load_nwb.py	54.38% <40.00%> (-0.53%)	⬇️
syncopy/io/nwb.py	83.54% <83.33%> (-0.67%)	⬇️

... and 4 files with indirect coverage changes

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