Introduce grouping classes into the vaccine hierarchy

[cthoyt]

Right now, the children of VO:0000001 (vaccine) imply several sub-hierarchies, including:

1. Vaccines organized by what disease they immunize against - Add VO:0005503 to group vaccines for diseases #620
2. Vaccines organized by what pathogen they immunize against - Add VO:0005504 - vaccine for pathogen #622
3. Vaccines organized by what platform they are developed on (e.g., DNA vaccines, RNA vaccines, etc.) - Add VO:0005505 - vaccine for platform #623
4. Several singletons

It would be great if we could add some mid-level hierarchy terms to make it easier to isolate terms from each of these groups. I will demonstrate for 1) vaccines by disease, then hopefully later we can tackle 2) and 3) which will have bigger diffs