(Re)implement the count_reporter plugin

[thilp]

As part of TIP/docs#330 “Introduce generic multi-level nested scenarios in Transformer”, we are breaking our count_reporter plugin, which assumes a fixed, 2-level structure of nested scenarios (ScenarioGroup then Scenario). The next step is to fix count_reporter so that it works well with the new structure, where ScenarioGroup and Scenario are merged and can be arbitrarily nested.

ℹ️ Imported from TIP/docs#387.