xcube server to return default color bar

[forman]

Is your feature request related to a problem? Please describe.

xcube server will return any color bar associated with a variable, independent whether the color bar name is known or not, e.g. "Plasma" instead of "plasma". xcube viewer will show a missing color bar image in this case:

Describe the solution you'd like

Let xcube server report the actual default color bar used in the case, i.e. "viridis".

Describe alternatives you've considered

[forman]

It will default to "viridis".