Implement VerticalGridTuple

[malmans2]

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I recently discovered namedtuple, and I think we could make use of it in many places.
For example, accessing sigma on the t grid doing sigmas.t seems more robust (and readable?) than sigmas[0].

We could use it for pps as well (e.g., place all tanh2 parameters in a named tuple and access those like this pp_tanh2.a2).

[codecov]

Codecov Report

Merging #56 (23787e2) into main (baeffab) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #56      +/-   ##
==========================================
+ Coverage   94.07%   94.13%   +0.06%     
==========================================
  Files           5        5              
  Lines         287      290       +3     
==========================================
+ Hits          270      273       +3     
  Misses         17       17              

Flag	Coverage Δ
unittests	94.13% <100.00%> (+0.06%)	⬆️

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Impacted Files	Coverage Δ
pydomcfg/domzgr/zco.py	91.83% <100.00%> (ø)
pydomcfg/domzgr/zgr.py	100.00% <100.00%> (ø)
pydomcfg/utils.py	97.26% <100.00%> (+0.03%)	⬆️

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[oceandie]

Defenitely more readable, I like it!