Netcdf metadata

Add iOPeNDAPVariable component for iterating through variables for an OPeNDAP resource.  And fix iOPeNDAPAttribute so it can be used to iterate through all attributes.

These were written a week or so ago when I was speccing the Moorings Harvester (so I could check actual attributes/variables used) but I never got around to adding them.

Xavier/Marty are interested in using these.