Linear focus tests, UI, and tweaks
Added a unit test for the linear focus algorithm.
Also, added a UI element https://photos.app.goo.gl/FQfX4Cc2iu7WeCJH7 which controls how far out (in multiples of step-size) the algorithms moves initially, before beginning its inward sweep.
Simplified some of the Linear Focus code that computes the initial position, and made a couple small tweaks to the algorithm.