code: @Nonnull
      public Person addPerson(@Nonnull String name, @Nullable String description) {
          return addPerson(Location.Unspecified, name, description);
      }
docs: Creates a person (with an unspecified location) and adds it to the model.
  
  @param name        the name of the person (e.g. "Admin User" or "Bob the Business User")
  @param description a short description of the person
  @return the Person instance created and added to the model (or null)
  @throws IllegalArgumentException if a person with the same name already exists