Import all classes in the toplevel __init__.py

[andlaus]

this would allow to import all classes directly in external code instead of having to know the sub-module where the class of interest is defined. example: With this,

from odxtools.diaglayer import DiagLayer

could be replaced by

from odxtools import DiagLayer

On the flipside, this increases the effort required for maintenance, because all classes provided by the project would need to be imported to __init__.py.