Draft: add lr-wpan energy-model

Preliminary merge request intended as request for comments.

Adds an energy model glue code to lr-wpan, that allows to simulate energy consumption analysis for lr-wpan devices.

One thing that should be noted: the accuracy of the model (obviously) depends strongly on the device parameters, and those can differ vastly in our experience. In fact, the model can only roughly approximate how such devices spend their energy - but it is a start.