Implement interventions on model rates

[pratikunterwegs]

This draft PR allows ODE models to include interventions on rate paramters. This:

1. Fixes intervention alters transmissibility #43 by allowing modification of the transmission rate $\beta$ (now available in the default model);
2. Fixes Add treatment #42 by editing the Vacamole model to allow modifying the hospitalisation rate $\eta$;
3. Defines an <intervention> super-class that can accommodate both age-stratified interventions on social contacts as well as uniform interventions on model parameters; associated documentation to be added;
4. Updates vignettes with new types of interventions;
5. Adds a basic vignette "Modelling interventions on model rates" showing the new functionality of interventions on model rates.