Add <epidemic> class

[pratikunterwegs]

This issue is to request a bare-bones <epidemic> class to hold model outputs, parameters, and other composable elements (interventions, vaccinations etc.). This is expected to help with predictability of the output format, and with methods such as a plot.epidemic() that conveniently plots an epidemic curve with any interventions, for easy visual comparisons of scenarios.

Further discussion copied from #160:

@Bisaloo:

• it allows us to have a stable and predictable output format (or class) for scenario modelling, which facilitates integration with downstream packages.
• should be interoperable with downstream packages

@TimTaylor:

• stochastic versus deterministic model output.
• Whether we want to capture warnings and errors (could require additional columns to store the information)

[pratikunterwegs]

Following discussions offline, this issue is suspended for now but is very likely to be taken up in the near future.