Add defaults to final_size()

[pratikunterwegs]

This PR fixes #180 by providing default values for all population parameters in final_size().

This allows final size calculation given only the epidemic $R_0$, assuming a homogeneously mixing population with full susceptibility, e.g.:

final_size(1.3)

[adamkucharski]

Looks good, thanks for adding. Handy to have this direct route to calculation for homogeneous population.