Improvements for use as module

Notably:

• Narrow the scope of implicit globals to while executing network config files.
• Support creation of Node objects directly from calling code.
• Provide a simpler interface for TorNet.createNetwork that takes a list of Nodes instead of a Node-generator-function.
• Add pyproject-managed "chutney" wrapper to be used instead of bespoke shell wrapper, when installed e.g. via pip.

Also:

• Add shellcheck and unit tests that were already in the repo to CI

This is primarily to support work on arti#590.