Scenario should inherit from data.table or data.frame

[pratikunterwegs]

This issue is to suggest that scenario objects should inherit from data.table or data.frame. This would have benefits, including:

1. Allow the use of methods for data.frames and data.tables, allowing users to easily build on top of scenarios, or to join and manipulate scenarios in ways that they are already accustomed to doing in their workflows with tabular data;
2. Put the data aspect of the scenario front and centre so that users know that scenario objects are essentially model outputs (in the form of data.frames), with some attached parameters.
   Other benefits could be added to comments on this issue.