Add topics for proportion of individuals susceptible

In SMH Flu round 1 25/26, some new targets were added:

• S0: initial proportion of individuals susceptible to influenza infection on the first day of a given simulation
• S0_A, S0_B, S0_AH1, S0_AH3: initial proportion of individuals susceptible to influenza A infection (respectively influenza B, influenza A/H1N1, influenza A/H3N2) on the first day of a given simulation

It might be interesting to add that topic to the ontology. I don't think we need to add more details so we might be able to instead add:

• susceptible population : A population of organisms that are all susceptible. http://purl.obolibrary.org/obo/APOLLO_SV_00000234 OR ount of susceptible population: The count of individuals in a susceptible population (empirical or virtual). http://purl.obolibrary.org/obo/APOLLO_SV_00000087

and tag also the entries with "flu", flu A and flu B if necessary.

Start by locating existing ontology entries for influenza targets and compare their structure with the proposed susceptible-population terms. Review the referenced APOLLO terms, then determine whether S0 and its influenza-specific variants need separate topics or tags. Done means the selected topics are represented consistently in the ontology.