Initial implementation of the scenario class

[pratikunterwegs]

This pull request includes developments related to the scenario class implementation sketched out in #3.

1. Scenario class structure modified to a four element list of model_function, parameters, data, and replicates.
2. A run_scenario function that populates the data column with model data (essentially creating a new scenario object, requires assignment).
3. A number of helper functions, some of them discussed in Design of helper functions #5 that access scenario parameters and scenario data, and aggregate scenario data by some grouping variables.
4. Fixes JSON import-export for scenario objects #7 by adding import-export functions to allow writing scenario objects (with or without data) to JSON files.
5. 100% test coverage reported from covr::package_coverage().