Fix diag nodiag

[manuschneider]

Elementwise operations between a diagonal DenseUniTensor and a non-diagonal DenseUniTensor are now possible:
Add, Add_, Sub, Sub_, Mul, Mul_, Div, Div_ were fixed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 35.31%. Comparing base (e87c3a2) to head (73a7931).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #716      +/-   ##
==========================================
+ Coverage   32.35%   35.31%   +2.95%     
==========================================
  Files         215      215              
  Lines       36363    33115    -3248     
  Branches    14597    13207    -1390     
==========================================
- Hits        11764    11693      -71     
+ Misses      22659    19508    -3151     
+ Partials     1940     1914      -26     

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