add fake vertical coordinate for bed thickness when sed Nbed is present

[rsignell-usgs]

As noted here: USGS-CMG/usgs-cmg-portal#195, bed_thickness is not appearing in the portal. This is because although these variables have a vertical dimension, they have no vertical coordinate variable, so the variables are not CF-compliant.

[rsignell-usgs]

We can fix this problem in NcML by adding a fake vertical coordinate variable for the bed:

     <variable name="Nbed" shape="Nbed" type="double">
      <attribute name="long_name" value="pseudo coordinate at seabed points"/>
      <attribute name="standard_name" value="ocean_sigma_coordinate"/>
      <attribute name="positive" value="up"/>
      <attribute name="formula_terms" value="sigma: Nbed eta: zeta depth: h"/>
      <values start="-1.0" increment="-0.01"/>
     </variable>

we need to have yaml2ncml write this into the NcML file when there are bed variables.

[ocefpaf]

This is done, right? Can we close it?