Implement lazy area weights

[schlunma]

Description

This is a simple workaround for lazy area weights until SciTools/iris#5658 is available.

Related to #674

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.30%. Comparing base (bbd307d) to head (e645a62).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2354   +/-   ##
=======================================
  Coverage   94.29%   94.30%           
=======================================
  Files         246      246           
  Lines       13540    13549    +9     
=======================================
+ Hits        12768    12777    +9     
  Misses        772      772           

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[valeriupredoi]

do we still need this in light of the new iris 3.9.0? If so, just say "duck" and I'll merge it 🦆

[schlunma]

🦆

[schlunma]

(the new feature is unfortunately not included in iris 3.9)

[valeriupredoi]

a 🦆 's been asked for!