End-users should fetch contained classes directly from sample

[andped10]

To give us freedom to do changes to the internal structure of the sample module without affecting end-users we need to list all exposed classes in the outermost __init__.py file.

Two examples:
The Multilayer class is implemented in sample.assemblies.multilayer should be available by executing:
from sample import Multiplayer

Layer is implemented in sample.elementals.layers.layershould be available by executing:
from sample import Layer

This change is started in: #87