Bidiagonal to Tridiagonal with immutable bands (#55059)
Using `similar` to generate the zero band necessarily allocates a
mutable vector, which would lead to an error if the other bands are
immutable. This PR changes this to use `zero` instead, which usually
produces a vector of the same type. There are occasions where `zero(v)`
produces a different type from `v`, so an extra conversion is added to
obtain a zero vector of the same type.
The following works after this:
```julia
julia> using FillArrays, LinearAlgebra
julia> n = 4; B = Bidiagonal(Fill(3, n), Fill(2, n-1), :U)
4×4 Bidiagonal{Int64, Fill{Int64, 1, Tuple{Base.OneTo{Int64}}}}:
3 2 ⋅ ⋅
⋅ 3 2 ⋅
⋅ ⋅ 3 2
⋅ ⋅ ⋅ 3
julia> Tridiagonal(B)
4×4 Tridiagonal{Int64, Fill{Int64, 1, Tuple{Base.OneTo{Int64}}}}:
3 2 ⋅ ⋅
0 3 2 ⋅
⋅ 0 3 2
⋅ ⋅ 0 3
julia> Tridiagonal{Float64}(B)
4×4 Tridiagonal{Float64, Fill{Float64, 1, Tuple{Base.OneTo{Int64}}}}:
3.0 2.0 ⋅ ⋅
0.0 3.0 2.0 ⋅
⋅ 0.0 3.0 2.0
⋅ ⋅ 0.0 3.0
```
