Implement the class InfectionState of the LCT model more efficiently

[lenaploetzke]

The idea here is to make the LCT model more efficient by doing more in the InfectionState class at compile time.

The ability to freely choose the number of subcompartments should not be lost.

We hope we can base it on the ODE flows.

Originally raised by @mknaranja in #915 (comment)

[mknaranja]

@lenaploetzke Please check the edited message is ok. Description should be top in issue.