An object of class "rate" represents a diver's
rate of ascent or descent, or the total time taken to
ascend or descend. Such objects are useful in describing
dives, especially in the function dive. The value returned by ascent represents an ascent rate
or ascent time. Exactly one of the arguments speed and
time should be given. If speed is given, this specifies
a fixed rate of ascent, in metres per minute. If time is given,
this specifies a fixed total time of ascent, in minutes.