Small changes to erddap netcdf export

[nicholas512]

• depth: no missing value (coordinate variable shouldn't have missing values).
  • Either make depth an auxiliary coordinate variable (makes it harder to plot in panoply)
  • or explicitly remove missing value since it won't be missing (only 1 station)
• profile_id : make sure this is included in the coordinates attribute for something http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2014/017394.html