Added comments to a seawater file

I like to be able to get the syntax for the code by typing “help
codename.” I added that functionality to this code.
Fixed bug in export_fig, renamed old export_fig

There was a bug in export_fig so that pdfs always had to be made using
the full pathname: export_fig(‘/Users/swarner/Documents/test.pdf’). A
bug was fixed so now I can save a pdf by calling export_fig with a
shortened pathname: export_fig(‘~/Documents/test.pdf’).

A HUGE thanks to Oliver Woodford who wrote export_fig, who responded to
my request AND fixed the bug in less than a day!