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

Not needed?  (There have been a few other changes involving spaces around operators that seemed gratuitous, and a few others where I wondered why spaces *weren't* added, but I don't want to spend more time nit picking.)