Absent trainee roster

for `property`s, use a leading underscore, since we are following the example in the django docs: https://docs.djangoproject.com/en/dev/topics/db/models/#model-methods

PEP 8 says that the leading underscore is a weak indicator of "internal use only" (http://legacy.python.org/dev/peps/pep-0008/#naming-conventions)