Use accelerometer to estimate which foot/feet is on the ground

[GreatAlexander]

Currently lower foot on Z axis (for calculating odom) is guessed from base_link, so it disregards if robot is unbalanced/tipping and actually touching the ground with a different foot.
This change should keep previous functionality intact, and help (probably) with unconventional motions like sidestepping (Currently odom calculation over-estimates sidestep motion).

As a fun fact, it could technically be consequently ported to any other TF frame (Arm crawling Marty anyone?)