Saves and reads the total mileage from the datacentre.

This change introduces the functionality of having a mileage parametre and saving it in the datacentre to counter the risk of data loss when crashing.

The node is initialise with the mileage stored in the mileage parametre and saves the current mileage to the datacentre every ~10 seconds (see README for details).

To set your mileage parameter to the current mileage on the robot before starting the node for the first time follow the instructions in the README file.