lookWithin(list)swRho(ctd), in which the first argument, which is
normally salinity may be an object that contains salinity plus
the other items that swRho expects to see as arguments. This
shorthand is very helpful in calls to the suite of sw functions. If
this first argument is an object of this sort, then the other arguments
are ignored except for one named eos, which is copied if
it is present.