Simplify subcommand example within argparse_example.py

[tleonhardt]

Simplify subcommand example within argparse_example.py to be a simple calculate command with add and subtract subcommands.

The original subcommand example was overly complicated. This replaces it with a much simpler one that is easier to digest and understand.

Also switched to using the cmd2.as_subcommand_to decorator.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.78%. Comparing base (61273d6) to head (68f6ab0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1498      +/-   ##
==========================================
+ Coverage   98.76%   98.78%   +0.02%     
==========================================
  Files          23       23              
  Lines        4925     4925              
==========================================
+ Hits         4864     4865       +1     
+ Misses         61       60       -1     

Flag	Coverage Δ
unittests	98.78% <ø> (+0.02%)	⬆️

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