BlockMatrixTensor

[max-vassili3v]

I have finished an implementation for the 4-tensor block matrix interface but it is still very slow. I spent a while experimenting with a lot of things to try and speed it up such as:

• populating with view(x, Block(i), Block(j)), which would sometimes lose sparsity.
• Using BlockedMatrix instead: this was probably the closest to a fast solution but there were issues with setindex! when the underlying structure was a Diagonal or BandedMatrix

The solution I currently have uses blocks() and mortar() primarily but this is still very slow. Could you please point me in the right direction?

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