Dsum

[cdwensley]

The direct sum operations are not working correctly, so some tests commented out.
Several of the algebra mappings created appear not to be single-valued: not at all sure why!
So this is a very provisional PR added to make the tests pass.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.92%. Comparing base (927c05a) to head (9bc9896).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/dsum-xmod.gi	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
- Coverage   72.09%   66.92%   -5.17%     
==========================================
  Files          10       12       +2     
  Lines        2584     2912     +328     
==========================================
+ Hits         1863     1949      +86     
- Misses        721      963     +242     

Files with missing lines	Coverage Δ
lib/dsum-xmod.gd	100.00% <ø> (ø)
lib/dsum-xmod.gi	22.15% <0.00%> (ø)

... and 1 file with indirect coverage changes

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