Checks how often a number sequence changes from decreasing monotonically to increasing monotonically (or vice versa).

## Details

The supplied vectors are assumed to be ordered by time.
Values do not have to be strictly monotonically in-/decreasing.
I.e., `c(0, 1, 1, 2)`

would return 0,
as \(x_n >= x_n-1\) is satisfied for \(2 <= n <= length(c(0, 1, 1, 2))\).

## References

Pfister, R., Tonn, S., Schaaf, M., Wirth, R. (2024). mousetRajectory: Mouse tracking analyses for behavioral scientists. The Quantitative Methods for Psychology, 20(3), 217-229. doi:10.20982/tqmp.20.3.p217