Usage
chop.dive(d, tim)
depths.dive(d)
is.dive(x)
is.gas(x)
is.rate(x)
print.gas(x, ...)
print.rate(x, ...)
print.summary.dive(x, ...)
print.summary.gas(x, ...)
rate(speed, time, up)
summary.dive(object, ...)
summary.gas(object, ...)
times.dive(d)
timetaken(start, finish, uprate, downrate)