ODE model functions should accept list of model component sets

[pratikunterwegs]

This issue is to request that model functions should accept lists of intervention and vaccination (collectively called model component sets).

• This is an extension of a use case laid out in this discussion, and would allow users to define a list of intervention sets and pass them to a single function call, making it more convenient to run multiple intervention scenarios.

intervention_set_01 = list(school_closure, mask_mandate)
intervention_set_02 = list(workplace_closure, mask_mandate, therapeutic_rollout)

model_default_cpp(
  population,
  intervention = list(intervention_set_01, intervention_set_02)
)

• Options for the new API for model functions is discussed in #160, with toy examples shown in this Gist;
• This change is initially expected to be rolled out for the Rcpp implementations of ODE models only; see #162;
• Intervention sets are expected to combine with parameter sets (see #166); for $N_i$ intervention sets and $N_p$ parameter sets, we expect $N_iN_p$ runs of the internal model function;
• Intervention sets may differ in their lengths (see code above);
• Following this change, model functions are expected to return a nested data.frame or data.table, with one row per parameter set and intervention set combination.