Create a tutorials exercise that demonstrates using PacketIn and PacketOut to read and write P4 register arrays

[jafingerhut]

For example, see the discussion on this issue #497, which may be a good start to anyone wanting to implement this in the tutorials repository. It may require changes to some of the Python controller code in the tutorials repository, too, ideally relatively small and focused changes, rather than rewriting it all.

They should probably add the --cpu-port <some-large-number-maybe-510> to all invocations of simple_switch_grpc, since that should be harmless for the existing exercise behavior.