Test compute numpy

This is almost ready for merging. I have one problem though, with the function on line 49:

```
@dispatch(BinOp, base, np.ndarray)
def compute_up(t, lhs, rhs, **kwargs):
    return t.op(lhs, rhs)
```

Had a lot of trouble figuring out how to call this function, figuring out what it wanted for inputs. Any feedback on this would be appreciated. 
There was a duplicate function: 

```
@dispatch(Selection, np.ndarray)
def compute_up(sel, x, **kwargs):
    return x[compute(sel.predicate, {sel.child: x})]
```

This one was better than that one, at least in my opinion.