Use get_node_vars for divB analysis rutines

[amrueda]

This addresses the comments in #2176. Although the current implementation is not causing extra allocations (because we work on a wrapped array), we rewrite these routines for consistency with the rest of the code.

[github-actions]

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[sloede]

LGTM, thanks!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.39%. Comparing base (a255fc0) to head (b1412e2).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2193   +/-   ##
=======================================
  Coverage   96.39%   96.39%           
=======================================
  Files         483      483           
  Lines       38333    38353   +20     
=======================================
+ Hits        36948    36968   +20     
  Misses       1385     1385           

Flag	Coverage Δ
unittests	96.39% <100.00%> (+<0.01%)	⬆️

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