Support indexing Tridiagonal/SymTridiagonal block matrices

[jishnub]

This uses diagzero instead of zero to obtain the zero elements, which works for matrix-valued elements as well. The following works after this:

julia> m = ones(2,2);

julia> T = Tridiagonal(fill(m,3), fill(2m,4), fill(3m,3))
4×4 Tridiagonal{Matrix{Float64}, Vector{Matrix{Float64}}}:
 [2.0 2.0; 2.0 2.0]  [3.0 3.0; 3.0 3.0]  …          ⋅         
 [1.0 1.0; 1.0 1.0]  [2.0 2.0; 2.0 2.0]             ⋅         
         ⋅           [1.0 1.0; 1.0 1.0]     [3.0 3.0; 3.0 3.0]
         ⋅                   ⋅              [2.0 2.0; 2.0 2.0]

julia> S = SymTridiagonal(fill(2m,4), fill(m,3))
4×4 SymTridiagonal{Matrix{Float64}, Vector{Matrix{Float64}}}:
 [2.0 2.0; 2.0 2.0]  [1.0 1.0; 1.0 1.0]  …          ⋅         
 [1.0 1.0; 1.0 1.0]  [2.0 2.0; 2.0 2.0]             ⋅         
         ⋅           [1.0 1.0; 1.0 1.0]     [1.0 1.0; 1.0 1.0]
         ⋅                   ⋅              [2.0 2.0; 2.0 2.0]

This was already supported for Diagonal/Bidiagonal, and this PR extends the feature to tridiagonal matrices as well.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.96%. Comparing base (62c8100) to head (ec09767).
Report is 5 commits behind head on master.

Additional details and impacted files

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##           master    #1218   +/-   ##
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  Coverage   91.96%   91.96%           
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  Files          34       34           
  Lines       15473    15473           
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  Hits        14229    14229           
  Misses       1244     1244           

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[jishnub]

Good to merge?