fix: data file sorting by creation time in plot.R

Just a small fix that lets the plotscript /usr/share/doc/co2monitor/examples/plot.R sort the data files by creation time, not just filename.

Known issues

Still, with this release, just don't unplug the device during warmup :-)

• The logging thread is not stopped when the device is unplugged during warmup. Instead, the thread does not exit at all, which is a problem. It also cannot be stopped from the dbus interface, co2monitor has to be stopped manually (call the quit method on dbus, or do a sudo systemctl stop co2monitor).