Add dual flag to SumSpace

[lkdvos]

This PR adds an explicit dual flag to SumSpace, which avoids the edge cases where isdual(SumSpace{S}()) is indeterminate and errors. This is a minimally breaking change, but could destroy stored data so I'm bumping this to v0.2.0.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.75%. Comparing base (523abc8) to head (39137a1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
+ Coverage   55.67%   55.75%   +0.07%     
==========================================
  Files          18       18              
  Lines        1419     1426       +7     
==========================================
+ Hits          790      795       +5     
- Misses        629      631       +2     

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