countstored tests
julia> A = Tensor(Dense(SparseList(Element(0.0))), [0.0 0.0 4.4; 1.1 0.0 0.0; 2.2 0.0 5.5; 3.3 0.0 0.0])
4×3 Tensor{DenseLevel{Int32, SparseListLevel{Int32, Vector{Int32}, Vector{Int32}, ElementLevel{0.0, Float64, Int32, Vector{Float64}}}}}:
 0.0  0.0  4.4
 1.1  0.0  0.0
 2.2  0.0  5.5
 3.3  0.0  0.0
julia> countstored(A)
5
julia> A = Tensor(SparseCOO{2}(Element(0.0)), [0.0 0.0 4.4; 1.1 0.0 0.0; 2.2 0.0 5.5; 3.3 0.0 0.0])
4×3 Tensor{SparseCOOLevel{2, Tuple{Int32, Int32}, Vector{Int32}, Tuple{Vector{Int32}, Vector{Int32}}, ElementLevel{0.0, Float64, Int32, Vector{Float64}}}}:
 0.0  0.0  4.4
 1.1  0.0  0.0
 2.2  0.0  5.5
 3.3  0.0  0.0
julia> countstored(A)
5
julia> A = Tensor(Dense(Dense(Element(0.0))), [0.0 0.0 4.4; 1.1 0.0 0.0; 2.2 0.0 5.5; 3.3 0.0 0.0])
4×3 Tensor{DenseLevel{Int32, DenseLevel{Int32, ElementLevel{0.0, Float64, Int32, Vector{Float64}}}}}:
 0.0  0.0  4.4
 1.1  0.0  0.0
 2.2  0.0  5.5
 3.3  0.0  0.0
julia> countstored(A)
12
julia> A = Tensor(SparseList(Dense(Element(0.0))), [0.0 0.0 4.4; 1.1 0.0 0.0; 2.2 0.0 5.5; 3.3 0.0 0.0])
4×3 Tensor{SparseListLevel{Int32, Vector{Int32}, Vector{Int32}, DenseLevel{Int32, ElementLevel{0.0, Float64, Int32, Vector{Float64}}}}}:
 0.0  0.0  4.4
 1.1  0.0  0.0
 2.2  0.0  5.5
 3.3  0.0  0.0
julia> countstored(A)
8

