Validate input for get method (and others potentially)

The get() method does not throw an error when using floating point numbers for the array indices. I know that it's kind of obvious to not do this, but I ran into a bug that was quite difficult to trace because the ndarray was returning data that seemed valid, but was just sometimes wrong.

Throwing an error when floating point numbers are used to access data seems quite helpful. I assume that this goes for the set method as well (and maybe others).