These functions extract data on detections, ignoring occasions when an
animal was not detected. Detections are ordered by occasion, animalID
and trap.
trap returns polygon or transect numbers if traps(object)
has detector type `polygon' or `transect'.
alongtransect returns the distance of each detection from the
start of the transect with which it is associated.
Replacement values must precisely match object in number of
detections and in their order. xy<- expects a dataframe of x and y
coordinates for points of detection within a `polygon' or `transect'
detector.