Added a service to map DeviceInstances
Added a service to map DeviceInstances

 - Adds a GetDeviceInstance call to com.victronenergy.Settings interface
 - This call is available on the object /Services/DeviceInstances
 - Actual deviceinstance allocations are stored in /Settings/DeviceInstances/{class}/{unique-id}/DeviceInstance
 - This means it is possible to see and modify allocations using dbus tools such as dbus-spy

API call: GetDeviceInstance(unique-id, class, preferred_instance_number)
Example call: GetDeviceInstance('HQ1234', 'battery', 30)
Returns: Assigned DeviceInstance, or -1 if error.

victronenergy/localsettings