Added _sparse_orthogonal_conditional

[sc336]

Draft PR for taking the GPflow components from this GPflux PR into GPflow. The work is based on this paper.

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PR type: bugfix / enhancement / new feature / doc improvement

Related issue(s)/PRs:

Summary

Proposed changes

• ...
• ...
• ...

What alternatives have you considered?

Minimal working example

# Put your example code in here

Release notes

Fully backwards compatible: yes / no

If not, why is it worth breaking backwards compatibility:

PR checklist

• New features: code is well-documented
  • detailed docstrings (API documentation)
  • notebook examples (usage demonstration)
• The bug case / new feature is covered by unit tests
• Code has type annotations
• Build checks
  • I ran the black+isort formatter (make format)
  • I locally tested that the tests pass (make check-all)
• Release management
  • RELEASE.md updated with entry for this change
  • New contributors: I've added myself to CONTRIBUTORS.md

[codecov]

Codecov Report

Base: 98.09% // Head: 98.04% // Decreases project coverage by -0.05% ⚠️

Coverage data is based on head (d6c38b8) compared to base (cce56f8).
Patch coverage: 40.00% of modified lines in pull request are covered.

❗ Current head d6c38b8 differs from pull request most recent head 66a0b8c. Consider uploading reports for the commit 66a0b8c to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2027      +/-   ##
===========================================
- Coverage    98.09%   98.04%   -0.06%     
===========================================
  Files           93       93              
  Lines         5263     5268       +5     
===========================================
+ Hits          5163     5165       +2     
- Misses         100      103       +3     

Impacted Files	Coverage Δ
gpflow/conditionals/conditionals.py	87.50% <40.00%> (-12.50%)	⬇️

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