Symmetry check in setindex! for Symmetric/Hermitian

[jishnub]

Currently, the following works unexpectedly:

julia> A = Symmetric(fill([1 2; 3 4], 3, 3))
3×3 Symmetric{AbstractMatrix, Matrix{Matrix{Int64}}}:
 [1 2; 2 4]  [1 2; 3 4]  [1 2; 3 4]
 [1 3; 2 4]  [1 2; 2 4]  [1 2; 3 4]
 [1 3; 2 4]  [1 3; 2 4]  [1 2; 2 4]

julia> A[1,1] = [1 2; 3 4]
2×2 Matrix{Int64}:
 1  2
 3  4

julia> A[1,1]
2×2 Symmetric{Int64, Matrix{Int64}}:
 1  2
 2  4

After this PR, the setindex! throws an error if the exact value cannot be used:

julia> A[1,1] = [1 2; 3 4]
ERROR: ArgumentError: cannot set a diagonal element of a symmetric matrix to an asymmetric value

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.70%. Comparing base (5d3d02a) to head (65b8538).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1317      +/-   ##
==========================================
- Coverage   93.72%   93.70%   -0.02%     
==========================================
  Files          34       34              
  Lines       15733    15734       +1     
==========================================
- Hits        14746    14744       -2     
- Misses        987      990       +3     

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