it would be nice if getSum has support for axis

[yujiosaka]

it should be something like bumpy.sum

> var Matrix = require('./dist/linear-algebra')().Matrix
undefined
> var m = new Matrix([[1, 2, 3], [4, 5, 6]])
undefined
> m.getSum()
21
> m.getSum(0)
{ data: [ [ 5, 7, 9 ] ], rows: 1, cols: 3 }
> m.getSum(1)
{ data: [ [ 6, 15 ] ], rows: 1, cols: 2 }