A character string indicating a unit. Case is ignored, so that e.g.
"dbar" and "DBAR" yield equal results. The following are recognized:
c("m-1", "dbar", "decibar", "degree", "degree_Celcius",
"degree_north", "degree_east", "ipts-68",
"its-90", "m/s^1", "m/s^2", "pss-78",
"umol/kg", "micromole/kg")