Some cleanups in stats

Playing with PyCharm, may as well get it to do something semi-useful....

This is mainly:

- PEP8 fixes
- changes in docstrings (imports) to conform to numpy doc standard
- removing extraneous parenthesis
- changing to in-place operations where possible

This changes the formula.  You still need parentheses around `N*(N-1.0)*(N-2.0)*(N-3.0)` to keep the entire expression in the denominator.  (Another function with no tests.  Sigh.)