Add Euthanized to icarDeathMethodType

[cookeac]

Add "Euthanized" enum to icarDeathMethodType.
Resolves #515

As an aside, we had a conversation about "Culled" vs. "Slaughtered" vs. "Euthanized".
Different systems will use this in different ways. "Culled" is often used as a reason for departures (unwanted animals are sold - culled). However in the context of deaths:

• Slaughter - typically means the animal has been killed for food (for people or other animals)
• Culled - may be used when animals are killed to stop disease spreading, or for biosecurity requirements
• Euthanized - describes an animal killed humanely because otherwise it would suffer (e.g from disease or injury)

We also talked about the challenges of documentation for enums, and I noted this conversation in the JSON Schema specification development: json-schema-org/json-schema-spec#57 (comment) (just interesting).