Fixed a number of issues with data from quarqd inf and nan values where
inserted as valid data points and thus destoying all plotting in the
realtime window and in later analysis.
The unit was used to distinguish between the entities, thus rpm was
erroneously used as a cadence, rpm is used as the unit for wheel
rotation and for cadence. This made the cadence useless together with a
PowerTap hub which reports both cadence and wheel rotation.
No error checking was performed on the received data, bad data is
ignored now.