Start adding typing annotations to ExactGP

[chrisyeh96]

This pull request does 3 things:

1. It starts adding typing annotations to ExactGP.

2. It standardize super() calls:

   # old
   super(ExactGP, self).foo()
   
   # new
   super().foo()
   

3. It fixes the error message in ExactGP.__call__() to clarify that it only needs train_inputs to be given. It is OK for train_outputs=None.