Added systemd unit file, to allow auto-start/stop of WebODM on systemd based Linux systems.

Pre-Requisites:
- Requires odm user
- Requires docker installed via system (ubuntu: apt-get install docker.io)
- Requires screen to be installed
- Requires odm user member of docker group
- Required WebODM directory checked out to /opt/WebODM
- Requires that /opt/WebODM is recursively owned by odm:odm
If all pre-reqs met, should be able to enable by doing:
systemctl enable /opt/WebODM/service/webodm.service
then reboot.
If you log in as odm user, service will run in a screen session called 'webodm'
So screen -r webodm will pull up console (CTRL+A, D) will close session
systemctl start webodm will start
systemctl status webodm will show process status
systemctl stop webodm will stop it.
It will be auto-started upon crash/error

service/webodm.service

@@ -0,0 +1,15 @@
+[Unit]
+Description=Start WebODM OpenDroneMap Service Container
+Requires=docker.service
+
+[Service]
+Type=forking
+User=odm
+Group=odm
+WorkingDirectory=/opt/WebODM
+ExecStart=/bin/bash -c 'screen -dmS webodm /opt/WebODM/webodm.sh start'
+ExecStop=/bin/bash -c '/opt/WebODM/webodm.sh stop'
+Restart=on-failure
+
+[Install]
+WantedBy=multi-user.target