Means of specifying function to call when all calibration samples are collected.

[damellis]

It can be useful to have one function that checks individual calibration samples and another that does the overall computation of calibration settings once all the samples are collected. When the last sample is collected, the system could aggregate warnings from both the individual sample function and the overall calibration function. This overall function should probably be optional, both for the case that there's only one calibration sample and the case where the calibration samples are independent.

See https://github.com/damellis/ESP/blob/master/Xcode/ESP/src/examples/user_accelerometer_gestures.cpp for a motivation (and work-around) for this issue.