Add 'time of introduction' to cohorts

[aornugent]

Calculating cohort.seed_survival_weighted requires calculating the probability of patch survival at time t, normalised by the probability of patch survival at birth (i.e. the time a cohort was introduced to a patch).

We currently record pr_patch_survival_at_birth when computing the initial conditions of a cohort. An alternative is to record the time of introduction (i.e. patch age) for each cohort, then use this existing pr_survival_conditional(time, time_of_introduction) method.