Add __orthogonal_indexing__ flag

[davidhassell]

Description

Add __orthogonal_indexing__ flag to the Dataset class.

Closes #171

Before you get started

• ☝ Create an issue](https://github.com/NCAS-CMS/pyfive/issues) to discuss what you are going to do

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 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.33%. Comparing base (edcc2d0) to head (08827b6).

Files with missing lines	Patch %	Lines
pyfive/high_level.py	75.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #182      +/-   ##
==========================================
- Coverage   77.33%   77.33%   -0.01%     
==========================================
  Files          15       15              
  Lines        2974     2982       +8     
  Branches      477      479       +2     
==========================================
+ Hits         2300     2306       +6     
- Misses        555      556       +1     
- Partials      119      120       +1     

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[bnlawrence]

This is consistent with the discussion