Validate append coordinates

[forman]

• A new configuration setting append_step can be used to validate the step sizes between the labels of a coordinate variable associated with the append dimension. Its value can be a number for numerical labels or a time delta value of the form 8h (8 hours) or 2D (two days) for date/time labels. The value can also be negative.

• The configuration setting append_step can take the special values "+" and "-" which are used to verify that the labels are monotonically increasing or decreasing.

Closes #20
Closes #21

[forman]

@pont-us, please add @clarasb as reviewer

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ee28dee) 99.82% compared to head (52da874) 99.83%.

❗ Current head 52da874 differs from pull request most recent head cc6303f. Consider uploading reports for the commit cc6303f to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #52   +/-   ##
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  Coverage   99.82%   99.83%           
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  Files          25       25           
  Lines        1149     1208   +59     
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+ Hits         1147     1206   +59     
  Misses          2        2           

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[clarasb]

I wrote you some comments.