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
└─ Dense [:,1:3]
   ├─ [:, 1]: SparseList (0.0) [1:4]
   │  ├─ [2]: 1.1
   │  ├─ [3]: 2.2
   │  └─ [4]: 3.3
   ├─ [:, 2]: SparseList (0.0) [1:4]
   └─ [:, 3]: SparseList (0.0) [1:4]
      ├─ [1]: 4.4
      └─ [3]: 5.5

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
└─ SparseCOO{2} (0.0) [:,1:3]
   ├─ [2, 1]: 1.1
   ├─ [3, 1]: 2.2
   ├─ ⋮
   ├─ [1, 3]: 4.4
   └─ [3, 3]: 5.5

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
└─ Dense [:,1:3]
   ├─ [:, 1]: Dense [1:4]
   │  ├─ [1]: 0.0
   │  ├─ [2]: 1.1
   │  ├─ [3]: 2.2
   │  └─ [4]: 3.3
   ├─ [:, 2]: Dense [1:4]
   │  ├─ [1]: 0.0
   │  ├─ [2]: 0.0
   │  ├─ [3]: 0.0
   │  └─ [4]: 0.0
   └─ [:, 3]: Dense [1:4]
      ├─ [1]: 4.4
      ├─ [2]: 0.0
      ├─ [3]: 5.5
      └─ [4]: 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
└─ SparseList (0.0) [:,1:3]
   ├─ [:, 1]: Dense [1:4]
   │  ├─ [1]: 0.0
   │  ├─ [2]: 1.1
   │  ├─ [3]: 2.2
   │  └─ [4]: 3.3
   └─ [:, 3]: Dense [1:4]
      ├─ [1]: 4.4
      ├─ [2]: 0.0
      ├─ [3]: 5.5
      └─ [4]: 0.0

julia> countstored(A)
8

