Scaling for partly initialized unit triangular

[jishnub]

After this, scaling unit triangular matrices with parents that don't have the diagonal initialized works.

julia> M = Matrix{BigFloat}(undef,2,2);
  
julia> M[1,2] = 3;

julia> U = UnitUpperTriangular(M)
2×2 UnitUpperTriangular{BigFloat, Matrix{BigFloat}}:
 1.0  3.0
  ⋅   1.0

julia> U * 2
2×2 UpperTriangular{BigFloat, Matrix{BigFloat}}:
 2.0  6.0
  ⋅   2.0

Probably needs #1350 for tests to pass.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.74%. Comparing base (3393398) to head (a290d4d).
Report is 11 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1351   +/-   ##
=======================================
  Coverage   93.74%   93.74%           
=======================================
  Files          34       34           
  Lines       15752    15747    -5     
=======================================
- Hits        14766    14762    -4     
+ Misses        986      985    -1     

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