Python organizational update

Yesterday we pushed a change that will encourage users to do

import hail as hl

instead of

from hail import *

Don’t use functions in your docs or code, use hl. The __all__ of the methods, expr, and genetics modules have all been added to the top-level namespace. If you write new functions, make sure to add them to the __all__ in expr. This also lets us split up functions into multiple files for organization more easily.