Add stepper classes for different threshold walking patterns

A simple stepper and a bisection stepper class are added by this commit.
The former reduces a value (the threshold) by one per update and the
latter compares the update value to its last value. If it is below zero,
half a step up is made, otherwise half a step down. This is repeated a
maximum of 100 times or the difference between the current and the last
value (threshold) is below 1.