Add Padding to Adapter

See #370.

We would like to add padding functionality to the Adapter in order to speed up computations when simulator outputs differ in size. For example:

```py3
adapter.pad("y", size=100, axis=-2)
```

when given `y.shape == (32, N, 2)` outputs `y.shape == (32, 100, 2)` for N < 100 and errors for N > 100.