amend b8921b9a89
* customize our multipliers. accuracy is a generic multiplier, mostly
depends on the environment temperature (see datasheet). sensitivity is
a ratio between the currently set mtreg value and the default
e.g. for mtreg 31, sensitivity is .45; for mtreg 69, sensitivity is 1.
* non-blocking pre(); since we have tick(), just wait until the reading
is available instead of stopping everything else in the sensor loop
* more specific error state when we are not ready to return a value