rotate_trajectory: Rotate trajectories
In MaxLev/CloseEnough: Analyze trajectories within a cellular context

Rotates all trajectories 'en bloc' using the centre of the image as origin.

1	rotate_trajectory(object, angle)

`object`	An object of class 'Traj'
`angle`	A numeric value corresponding to the desired angle of rotation. If positive, trajectories are rotated clockwise. If negative, they are rotated counterclockwise.

## Upload image and trajectories
data(cellmask)
data(trajectory)

rotatedtraj <- rotate_trajectory(trajectory, 90)
## Display image (first frame only)
EBImage::display(cellmask, method = "raster")

## Overlay trajectories in black
ntraj = nlevels(trajectory["trajectory"])
for (i in 1:ntraj) {
traj_i <- subset(trajectory["data"], trajectory == i)
lines(traj_i$x, traj_i$y, type="l", col = "black")
}
## Overlay rotated trajectories in blue
nrotatedtraj = nlevels(rotatedtraj["trajectory"])
for (i in 1:nrotatedtraj) {
traj_i <- subset(rotatedtraj["data"], trajectory == i)
lines(traj_i$x, traj_i$y, type="l", col = "blue")
}