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.