Extra output for booster analysis

[slwu89]

For analyses performed here https://github.com/mrc-ide/covid_vaccine_dose_analysis we need:

• [x] output incidence (S->E transitions) per time step. Some type of utility function to convert inc/dt to inc/day would be nice. (https://github.com/mrc-ide/safir/commit/5c6fac3f5b7c5cc81ecbbfc751baf495cc198249)
• [x] output vaccine dose uptake stratified by age, just cross-tabulate vaccine dose integer variable with age variable. (https://github.com/mrc-ide/safir/commit/07a8527a61b42ea1829ea91d3304b744637827b4)

[slwu89]

both are fixed in a branch, will close when merged into main.

[slwu89]

Done #30