define and test minimum dependencies

[mathause]

• closes add & test minimal dependecies  #124

Defines the minimum dependencies as per the xarray interpretation of NEP29 and adds a test environment for them. Done using pip now. What is not included here is a min-deps check. Need to manually check but I think that's ok.

Let's see if that works.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.65%. Comparing base (2f10049) to head (f7be61d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #125   +/-   ##
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  Coverage   99.65%   99.65%           
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  Files           5        5           
  Lines         293      293           
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  Hits          292      292           
  Misses          1        1           

Flag	Coverage Δ
unittests	99.65% <ø> (ø)

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