spacing and docstrings
move some table functions to collections.py
move reductions from table.py to reductions.py
move relabel from table to expr
move by from expr to split_apply_combine
move join from table to collections
move method dispatches from table to broadcast, expressions
TableSymbol is just a function, not a class
remove TableExpr
remove reference to expr.table
move apply, add .columns for backwards compatability