Convert between BlockDiagonals of different block types

[mzgubic]

Helps with #102

The idea is that if

julia> convert(Matrix, Symmetric(rand(2, 2)))

is allowed, then so should convert between BlockDiagonals where the blocks are of these types, e.g.

julia> convert(BlockDiagonal{Float64, Matrix{Float64}}, BlockDiagonal([Symmetric(rand(2, 2))]))

[codecov]

Codecov Report

Merging #105 (650b6ed) into master (b2ee90a) will decrease coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #105      +/-   ##
==========================================
- Coverage   96.11%   95.98%   -0.14%     
==========================================
  Files           5        5              
  Lines         335      324      -11     
==========================================
- Hits          322      311      -11     
  Misses         13       13              

Impacted Files	Coverage Δ
src/blockdiagonal.jl	86.76% <100.00%> (+0.40%)	⬆️
src/linalg.jl	95.34% <0.00%> (-0.21%)	⬇️
src/base_maths.jl	100.00% <0.00%> (ø)

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