Reconsider two-stage vaccination for Vacamole

[pratikunterwegs]

This issue stems from #227 - time-varying, age-stratified vaccination is more challenging to implement for a multi-stage vaccination regime as in Vacamole ($S \rightarrow V_1 \rightarrow V_2$). Furthermore the compartment $V_1$ is simply a holding compartment allowing transitions to $V_2$, but is not modelling as conferring any protection from the first dose.

Alternatives to consider:

1. $V_1$ is removed and there is only one vaccination compartment which works well with time-varying vaccination;
2. $V_1$ is retained and confers (leaky) protection allowing dose allocation scenarios to be modelled.