Define a fork with N alternative sub-trajectories.

branch(.trj, option, continue, ...)

## Arguments

.trj
the trajectory object.
option
a callable object (a function) which must return an integer between 0 and N. A return value equal to 0 skips the branch and continues to the next activity. A returning value between 1 to N makes the arrival to follow the corresponding sub-trajectory.
continue
a vector of N booleans that indicate whether the arrival must continue to the main trajectory after each sub-trajectory or not.
...
N trajectory objects describing each sub-trajectory.

## Value

Returns the trajectory object.