This is the function that powers all of the tour animations. If you want
to write your own tour animation method, the best place to
start is by looking at the code for animation methods that have already
implemented in the package.
Hadley Wickham, Dianne Cook, Heike Hofmann, Andreas Buja
(2011). tourr: An R Package for Exploring Multivariate Data with
Projections. Journal of Statistical Software, 40(2), 1-18.
http://www.jstatsoft.org/v40/i02/.