(CRUD)
###get all users
GET /courses/:course/problemset/:problemset/users
###get/update problem for a user
GET /courses/:course/problemset/:problemset/users/:user/:problem
PATCH /courses/:course/problemset/:problemset/users/:user/:problem
###add/remove problem
POST /courses/:course/problemset/:problemset/:problem
DELETE /courses/:course/problemset/:problemset/:problem
###add/remove users
POST /courses/:course/problemset/:problemset/:problem
DELETE /courses/:course/problemset/:problemset/:problem
###get grade for a user
GET /courses/:course/problemset/:problemset/user/:user/grade
###get past answers for a user
GET /courses/:course/problemset/:problemset/user/:user/pastanswers
###regrade a problem
GET /courses/:course/problemset/:problemset/regrade/:problem