Make BoundaryDict non-iterable

[adtzlr]

This is not used at all and the explicit BoundaryDict.items() still works. As the return value of the loadcases have changed, this will throw an error for old scripts which contain boundaries, loadcase = fem.dof.uniaxial(field).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.40%. Comparing base (db15698) to head (610cc7e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1023   +/-   ##
=======================================
  Coverage   97.40%   97.40%           
=======================================
  Files         184      184           
  Lines        7015     7018    +3     
=======================================
+ Hits         6833     6836    +3     
  Misses        182      182           

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