create a Systemd service file.

This file is used to start the program at startup. It, also, restarts the
program if it crashes or throws an exception. The usual scenario which
causes the program to crash is the 'max. requests exceeded' exception in
the 'requests' module (Python). When that happens, this 'restart' service
has a 60 seconds wait period -- which acts as a 'backoff' period.

You will need to copy the .service file to the appropriate systemd
location, 'daemon-reload' systemctl, enable the service and start the
service. You will need to look-up how to do that -- this info. is beyond
the scope of this commmit.

light-meter.service

@@ -0,0 +1,16 @@
+[Unit]
+Description=Light-Meter service for Return to Ritherdon project by Nicola Ellis
+After=network.service
+
+[Service]
+Type=simple
+ExecStart=bash /home/rtrp/repos/light-meter/startup.sh
+WorkingDirectory=/home/rtrp/repos/light-meter
+StandardOutput=inherit
+StandardError=inherit
+Restart=always
+RestartSec=60
+user=rtrp
+
+[Install]
+WantedBy=multi-user.target