Fix 1-pixel wide black line in generated +Y cube face image

[emzo]

Fixes https://github.com/indus/kubi/issues/3

There are multiple ways to acheive this in numpy, but this seems to be the most performant. Having said that, I'm no expert with numpy!

[emzo]

The view variable is just a another view of the ls NumPy array - changes made to view are reflected in ls since they share the same data. It's just easier to add a constant value to each element in the array when you have an array of shape (1,10) rather than (,10) like ls has.