Add variant-specific transmission parameters

[ben18785]

All the vignettes should be using Covid-19-relevant parameters. Rather than redefining them each time, I think it would be better to creating a function like covid_transmission_parameters(variant="base", age_structured=FALSE) which returns:

list(kappa, gamma, mu)

where gamma and mu are either numbers or vectors dependent on whether the model is age-structured. The documentation would contain references to where these parameters come from.

Also, by allowing the parameters to be variant-specific, we could be clear about which variant we are examining.

[ben18785]

As part of this, we probably want to update the gamma and mu parameters to use a more sensible infectious period duration, as in here.

[ben18785]

This is in there now through #69