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

Wirth, R., Foerster, A., Kunde, W., & Pfister, R. (2020). Design choices: Empirical recommendations for designing two-dimensional finger tracking experiments. Behavior Research Methods, 52, 2394 - 2416. doi:10.3758/s13428-020-01409-0