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])
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])
SparseCOO{2} (0.0) [:,1:3]
├─ [2, 1]: 1.1
├─ [3, 1]: 2.2
├─ [4, 1]: 3.3
├─ [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])
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])
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

