Matrix sum along a dimension is defined to return a `MathScalarType`, but might return higher dimension

**Describe the bug**
When summing over an axis of a matrix, we always lose just one dimension. If the matrix is 2D, we'll get a 1D array. If the matrix is 3D, we'll get a 2D matrix.
What happens now, is that `math.sum` declares that it returns a `MathScalarType`, instead of optionally returning a matrix.

**To Reproduce**
```
const mat = math.matrix([[1,2,3],[4,5,6]])
console.log(math.sum(mat, 0))

> DenseMatrix { _data: [ 5, 7, 9 ], _size: [ 3 ], _datatype: undefined }
```