Changed rover mesages from dictionary to named tuple

[ottopasuuna]

I thought that implementing rover messages as dictionaries was not optimal as a message only has one key and one data element. Finding what the message's key is is awkward, and makes things like message routers (like the USBServer) anoying to implement. I changed messages to be a named tuple, which is in the standard python collections library and is like a regular tuple, but you can access the elements by name.

Example:

 my_message = RoverMessage("TestKey", [1, 2, 3])
 my_message[0] == my_message.key # True
 my_message[1] == my_message.data # True

Note that you never have to create instances of RoverMessage. This is done by the RoverProcess class. You only have to make sure you access message elements properly in message trigger methods.

List of changes:

• Added RoverMessage named tuple with fields 'key' and 'data'
• Updated core to use RoverMessages instead of dictionaries
• Updated ExampleProcess, DriveProcess, StateManager, and test processes. Other modules will be fixed as time goes on (If you wrote a process that receives messages, you should update it 😉 ).