Bug Fixes on MCNP SurfSrc Class (niss variable improper assign to long, wrong labels on tracks); more PEP8 on mcnp.py

The track records should have these labels (in this order)

1. nps
2. bitarray
3. wgt
4. erg
5. tme
6. x
7. y
8. z
9. u
10. v
11. cs

Direction cosine `w` is backed out from `u` and `v`.

Also @scopatz, we had a discussion about optional `max_tracks` on the `print_tracklist()` function, but I reverted a commit and it got lost until I realized that I needed it (when finally using it).  The rest is just complaints I found during flake8 checks.

Unit tests pass on my slow laptop but should be double-checked.

```
Ran 662 tests in 11.232s

OK
```