Signal attributes of a `capthist' object are stored in a dataframe
called the signalframe. This has one row per detection. The signalframe
includes the primary field `signal' and an unlimited number of other
fields. To extract the signal field alone use the signal method.
These functions extract data on detections, ignoring occasions when an
animal was not detected. Detections are ordered by occasion, animalID
and trap.
Replacement values must precisely match object
in number of
detections and in their order.