Feature/clarify extra prefixes behavior 97

[jagerber48]

Resolves #97

In the end this clarification ended up simply being the removal of

global_add_c_prefix
global_add_ppth_form
global_add_small_si_prefixes
global_reset_iec_prefixes
global_reset_parts_per_forms
global_reset_si_prefixes

and some additional documentation and tests making the expected behavior more clear.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d3e09c9) 100.00% compared to head (bcc100b) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #113   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           23        23           
  Lines         1110      1096   -14     
=========================================
- Hits          1110      1096   -14     

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