Cleanup arraypad.py (PEP8, PEP257) and correct docstrings

Working on some speed improvements for `pad` (try it on a (40, 40, 40, 40) ndarray), but decided to submit this cleanup as a separate PR in the meantime.

There were a few minor docstring corrections converting `]` into `)` or vice versa. I believe everything else is PEP8 or PEP257 checks.