Allow for variables computed on-the-fly

[forman]

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

We'd like to add computed variables to data cubes published by xcube serve.
We already support computed cubes in xcube serve configurations, but that creates entirely new cubes (e.g. resampled in time). We now need to augment existing cubes by newly computed variables.

Describe the solution you'd like

Add a section Augmentation into dataset descriptor, e.g.:

Datasets:
  - Identifier: abc
    ...
    Augmentation:
      Path: compute_new_vars.py

The Augmentation entry may later be extended to also support variables computed from simple expressions:

    Augmentation:
      Variables: 
          - Name: conc_chl_g
            Expression: conc_chl / 1000
            Attributes:
               - units: "g / m^3" 
          - ...          

[forman]

Done.