Fully support lower levels by the CheckMK graphing system

The CheckMK graphing system supports the use of scalars in graphs for warning/critical levels, and the use of conditions for perfometers using the "<var>:warn" and "<var>:crit" notation. Unfortunately, this currently only works for upper levels, not for lower levels.

For services like battery capacity, signal/noise ratio and sometimes also temperature, exceeding lower levels is of much more concern than the upper levels. Support of lower levels by the graphing system opens the possibility to visually enhance the data view for these kind of services. For instance by showing the lower scalars and/or by creating perfometers that are coloured green/yellow/red according to the service state. This is now only possible for services that use upper levels.