Generalized eigen for complex Hermitian BandedMatrix

[jishnub]

julia> A = Hermitian(brand(ComplexF64, 6, 6, 2, 2));

julia> B = Hermitian(BandedMatrix(0=>ComplexF64.(1:6)));

julia> λb = @btime eigvals($A, $B); # This PR
  3.415 μs (13 allocations: 1.73 KiB)

julia> λ = @btime eigvals($(Hermitian(Matrix(A))), $(Hermitian(Matrix(B))));
  7.220 μs (10 allocations: 1.98 KiB)

julia> λb ≈ λ
true

[codecov]

Codecov Report

Patch coverage: 98.87% and project coverage change: +2.04 🎉

Comparison is base (c32e3e0) 86.73% compared to head (0bfb10c) 88.78%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #360      +/-   ##
==========================================
+ Coverage   86.73%   88.78%   +2.04%     
==========================================
  Files          24       24              
  Lines        3378     3424      +46     
==========================================
+ Hits         2930     3040     +110     
+ Misses        448      384      -64     

Impacted Files	Coverage Δ
src/symbanded/symbanded.jl	92.40% <ø> (-0.62%)	⬇️
src/symbanded/SplitCholesky.jl	67.40% <92.30%> (+0.99%)	⬆️
src/lapack.jl	89.09% <100.00%> (+0.97%)	⬆️
src/symbanded/bandedeigen.jl	88.34% <100.00%> (+7.82%)	⬆️
src/symbanded/tridiagonalize.jl	98.73% <100.00%> (ø)

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