separate executable scripts factoring out common code into modules

[jack-oquin]

Make modules in src/ with code common to the various nodes.

Move the nodes themselves into scripts/ (standard location). Each one should just import and then invoke an appropriate module function.

[jack-oquin]

I recommend that the documented node names not end in ".py". The fact that they are written in Python is an implementation detail. Users should not care.

That change (if accepted) should be made with "tick-tock" backwards compatibility. For one release we would support the old names, issuing a deprecation warning if they are invoked. In the following release the old names would be removed.

[jack-oquin]

Having most of the logic in Python modules generally makes it easier to write unit tests (see issue #16).