Add I19 transmission system business logic

This will start from validated specification class instances,
build absorber sub-systems (and the entities within)
and be ready to respond to absorption related requests.

## Acceptance Criteria
* That a device exists representing the transmission system, which can be directly invoked from plans.

Start from the validated specification class instances and trace how existing device definitions are made available to plans. Identify the transmission-system entry point, then follow the absorber sub-systems and their entities to understand the absorption-related requests. Done means a device representing the transmission system can be directly invoked from plans.

Written by the indexing model from the issue text.