Improvement: Step 2 tutorial and get_num_global_elements to mesh.hxx

[Vyp3er]

…mesh.hxx.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.28%. Comparing base (66cd94f) to head (54f48b6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2371   +/-   ##
=======================================
  Coverage   82.28%   82.28%           
=======================================
  Files         125      125           
  Lines       20701    20701           
=======================================
  Hits        17033    17033           
  Misses       3668     3668           

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