ODE model functions should accept vectors of infection parameters

[pratikunterwegs]

This issue is to request that model functions should accept vectors of infection parameters.

1. The expected use case is laid out in this discussion, and in Integration with input distributions and potential for fitting #20;
2. Options for the new API for model functions is discussed in Possible API for models #160, with toy examples shown in this Gist;
3. This change is initially expected to be rolled out for the Rcpp implementations of ODE models only; see Reconsider R implementations of ODE models #162;
4. Parameter input vectors are expected to follow Tidyverse recycling rules;
5. Following this change, model functions are expected to return a nested data.frame or data.table, with one row per parameter combination.