Refactoring in focus.cpp to separate this algorithm from the other algorithms.
Uses a polynomial fit to estimage when the first pass is done, and how far the current sample is from the minimum.
Improved logging.
Improved plotting when running the algorithm.
Algorithm applies to timer-based focusers as well as position-based ones.

Using the simulator, in the focus tab select the linear focus algorithm,
set the step size to about 1000. Set the MaxTravel to be large (e.g. 100K),
probably use "Full field" (that's less noisy). Should autofocus starting from 50K
(the solution will be about 38K). Should also find a solution if you start too low (20K),
and too high (75K) though would probably exceed the max number of steps (30) if you start higher.