Fix/field components

[pawel-wolff]

Description

This PR makes a few minor fixes in Field components:

• ad-hoc registered new type levels: the attribute "positive" is now "" instead of None, which, when used as an attribute in xarray, was breaking serialisation into netcdf
• xr_engine.coord.LevelCoord uses Field.vertical.cf() instead of creating the same dictionary on the spot that is feeding the coordinate attributes (it is a code refactoring in a sense, not a behaviour change)
• fix of a simple refactoring bug in the Field.proc component

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.43%. Comparing base (a0d6492) to head (5b95afb).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1029   +/-   ##
========================================
  Coverage    82.43%   82.43%           
========================================
  Files          237      237           
  Lines        16683    16683           
  Branches       806      806           
========================================
  Hits         13752    13752           
  Misses        2694     2694           
  Partials       237      237           

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