Make unitary matrices in svd/eigen of Diagonal be unitless

[dkarrasch]

Follow-up to #1150 and the discussion therein.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.89%. Comparing base (97a712f) to head (530fcee).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/diagonal.jl	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1155      +/-   ##
==========================================
- Coverage   91.92%   91.89%   -0.04%     
==========================================
  Files          34       34              
  Lines       15353    15355       +2     
==========================================
- Hits        14113    14110       -3     
- Misses       1240     1245       +5     

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