Similar SparseMatrixCOO

[MaxenceGollier]

There is a problem when trying to allocate memory for a similar SparseMatrixCOO type.
For any matrix,

A = SparseMatrixCOO(matrix)
B = similar(A)

and typeof(B) returns Matrix type. I guess this is due to the fact that SparseMatrixCOO <: Matrix and that similar( A :: SparseMatrixCOO) is not implemented ? I can add a PR with this method if this makes sense.

[dpo]

A PR would make sense.

[dpo]

Implemented in #40.

[MaxenceGollier]

With #40, when calling

B = similar(A)

on a matrix A of type SparseMatrixCOO, we don't copy the sparsity pattern of A into B (this obviously leads to errors and is not what SparseArrays.jl does for example).
I solved this issue in a new PR #46.