OneIndexedArray Slicing Bug

This oopsie is almost certainly my fault:

```
>>> import cctk
>>> import numpy as np
>>> arr_np = np.array([1,2,3])
>>> arr_cctk = cctk.OneIndexedArray(arr_np)
>>> arr_cctk
OneIndexedArray([1, 2, 3])
>>> arr_cctk[:]
OneIndexedArray([1, 2])
```