Check "level" coordinate based on each variable

[agstephens]

Check "level" coordinate based on each variable:

• this should be in our level definition: 10m, 1.5m, 850hPa etc.
• some will not have one.

[agstephens]

For some datasets:

need to check a vertical coordinate can be found
as a dimension
or, as a connection through the ":coordinates" attribute