Add Immigration function (like births) as part of Human Demography

[smitdave]

Set up Immigrantion as an option with dHdt

[smitdave]

Immigration defines an increase in population by infection state for every stratum. It's hard to think of a generic way of handling this:

• dHdt works on every state the same way, so it can handle emigration but not immigration

• Births only happen in new born strata into certain states

The right solution seems to be something like:

dSdt = .... + S_immigration(t)