add swRrho()
swRrho doc improvement
read.ctd.woce() now understands an arctic format (not documented)
read.ctd.woce() improvements
add mapLongitudeLatitudeXY
read.ctd.woce(): recognize W lon and S lat in .ctd files
read.ctd.woce(): fix minutes-old typo
standardDepths() added
gradient: remove two unnecessary (testing) lines
interpBarnes(): new arg 'trim'
interpBarnes(): add 'interpolateToData' argument
interpBarnes() doc improvement
interpBarnes(): return zd = interpolated value at data
read.ctd.sbe() now decodes e.g. "** Depth (m): 3447 "

Yet another format for a human-entered water depth.  Sigh.
read.ctd() improved to handle more formats for lat-lon etc

I also added two more test files in extdata, and plan to add one for
each new ctd format that oce can read, so that they will form a running
test of backwards compatibility.
interpBarnes(): 'trim' argument permits NA in data
swRrho(): give NA if too few data in profile
swRrho(): give NA if fewer than four data in profile
docs: shorten a lot of title lines
decode dates in Beaufort Sea .cnv files