Added some comments in commands.c/h, callbacks.c/h

- Added a comment saying that we may not need the ID parameter on the MessageType Struct anymore.
- Removed calls of the nature: "MessageTypes[TYPE_ID].ID" and replaced them with "TYPE_ID" because it is the same thing..
- Added function comments in callbacks.c
- Simplified the callbacks.c by having a more generic setval, getval, respval callback function instead of a billion of them for each PID value.