added realization dimension

[cehbrecht]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes issue #xyz
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• HISTORY.rst has been updated (with summary of main changes)
• I have added my relevant user information to AUTHORS.md

• What kind of change does this PR introduce?:
  New feature:

this PR adds a new dimension realization_index with standard name realization to the known coordinates.

The change is necessary to work with CMIP6 decadal datasets. In Copernicus we want to use the average_over_dim operator to merge the 10 realization members of each CMIP6 decadal dataset.

The notebook example with concat (new) + average operator:
https://nbviewer.org/github/roocs/rooki/blob/master/notebooks/demo/demo-rooki-concat.ipynb

• Does this PR introduce a breaking change?:
  no

• Other information:

[Zeitsperre]

I'm not super familiar with ROOCS-utils, but I don't see anything here that should be breaking!