Add SparseMatrix block addition feature (static+dynamic calls)
Add SparseMatrix block addition feature (static+dynamic calls)

SparseMatrix::block<n, m>(rowStart, colStart) +=
SparseMatrix::block(rowStart, colStart, n, m) +=
and for completeness, also implement the following feature
SparseMatrix::block(rowStart, colStart, n, m) =

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