plotClusterTrajectories() with ribbon for trajectory range

[niekdt]

Add an option to plotClusterTrajectories() to enable a ribbon plot for useful ranges (e.g., SD, standard error, quartiles, min-max)

This could be achieved through the existing trajectories argument.

Whereas trajectories = TRUE shows the original trajectories, other options could be enabled through:
plotClusterTrajectories(trajectories = "range") for min-max
plotClusterTrajectories(trajectories = "sd") for standard deviation
plotClusterTrajectories(trajectories = "se") for standard error
plotClusterTrajectories(trajectories = "quartile") for interquartile range
And arbitrary percentile intervals using the format \d{1,2}pct
e.g., plotClusterTrajectories(trajectories = "80pct") for 80th percentile range

The benefit of specifying the options through character strings is that this could enable a subtitle describing what range the ribbon presents. With arbitrary function input for trajectories, this would get more difficult.