DiagonalTensorMap performance and convenience specializations

[lkdvos]

Introduce a fast path for singular/eigen value calculations, implement left and right in-place multiplication methods, and add convenience constructors for DiagonalTensorMap.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.82%. Comparing base (0c73bd2) to head (ffac196).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #249      +/-   ##
==========================================
+ Coverage   82.65%   82.82%   +0.16%     
==========================================
  Files          43       43              
  Lines        5593     5601       +8     
==========================================
+ Hits         4623     4639      +16     
+ Misses        970      962       -8     

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[lkdvos]

Failures on nightly are unrelated, this should be good to go.
The main reason is that we are calling cond(S) in PEPSKit, where S comes from an SVD, and the fallback definition is taking a surprisingly long time for that.