Introduce a small `TreeObject` class that is the representation of
a tree during its construction. It supports calculating its treesum
as well initialize the new tree with an existing one.
Import modules between files using the syntax `from . import foobar`,
renaming what used to be `FooBar` to `foobar.FooBar` when moved to a
separate file.
In __init__.py only import what is meant to be public API.
Signed-off-by: Tom Gundersen <teg@jklm.no>
