Use isinstance instead of boolean checks

[charmoniumQ]

Instead of request.requiresOnlineAPIAccess(), it's better to do isinstance(request, OnlineRoutineRequest) for three reasons:

1. It helps you know that the request will have the attributes that OnlineRoutineRequests should have, e.g. getCredentials().
2. It helps static type-checkers know the type of that variable. A future PR will implement Mypy type annotations for this codebase, and the isinstance checks tell Mypy that the type of that variable, allowing the code to typecheck successfully.
3. It results less code (this commit is net negative LoC)

This PR is based on #31, #32, #34, and #35.