Forward _tri(l/r)div! to parent of triangular

[jishnub]

These loop over the stored triangular halves, so we may forward the operations to the parents. These are also internal methods that are not applied to unit triangular matrices, so we restrict these to UpperTriangular and LowerTriangular matrices in this PR. This makes it straightforward to forward the operations to the parent.

[dkarrasch]

What happens in the [unit] cases that you removed here?

[jishnub]

The unit triangular methods re-wrap the parents in a standard triangular wrapper, and call the corresponding methods. However, it seems more work needs to be done here to avoid errors from partly initialized parents. In any case, these methods are not called in the unit triangular cases.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.88%. Comparing base (3393398) to head (7a5cfca).
Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1348      +/-   ##
==========================================
+ Coverage   93.74%   93.88%   +0.14%     
==========================================
  Files          34       34              
  Lines       15752    16084     +332     
==========================================
+ Hits        14766    15100     +334     
+ Misses        986      984       -2     

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