Direct imports of `snmachine` classes and methods.

[rbiswas4]

Imports of snmachine classes and methods that users use should not depend on the internal organization of the code and should be accessible from an snmachine import, rather than requiring to specify the module within snmachine where these objects are located.
This requires

• changing the __init__ file to import such methods
• Unit tests should be for there such relevant methods and classes (which we mostly have). The tests should be changed to use just the snmachine imports