Support for Opaque datasets

[bnlawrence]

Description

Added support for opaque datasets, or at least those written by h5py. Would need some real test files written by someone else to check full support (but I am hopeful it would work).

Closes #88
Closes #70

Checklist

• This pull request has a descriptive title and labels
• This pull request has a minimal description (most was discussed in the issue, but a two-liner description is still desirable)
• Unit tests have been added (if codecov test fails)
• Any changed dependencies have been added or removed correctly (if need be)
• If you are working on the documentation, please ensure the current build passes
• All tests pass

[codecov]

Codecov Report

❌ Patch coverage is 90.32258% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.05%. Comparing base (c962394) to head (7cb0531).
⚠️ Report is 164 commits behind head on main.

Files with missing lines	Patch %	Lines
pyfive/datatype_msg.py	81.81%	1 Missing and 1 partial ⚠️
pyfive/h5t.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #114      +/-   ##
==========================================
+ Coverage   73.32%   74.05%   +0.73%     
==========================================
  Files          11       11              
  Lines        2575     2598      +23     
  Branches      401      406       +5     
==========================================
+ Hits         1888     1924      +36     
+ Misses        580      565      -15     
- Partials      107      109       +2     

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[kmuehlbauer]

@bnlawrence This looks like a nice addition. I've added a couple comments and suggestions.

[kmuehlbauer]

Great work!

[valeriupredoi]

very many thanks @bnlawrence and @kmuehlbauer 🍻