Digraph decode/encode

[anujthak]

Addressing the latter part of #47 , makes it so that it is now impossible to encode a digraph into an improper format(ex: a graph with multiple edges into Digraph6) and also recommends alternatives for a proper encoding in the error message.

[codecov]

Codecov Report

❌ Patch coverage is 52.63158% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.32%. Comparing base (5fd889a) to head (69fe761).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
gap/io.gi	52.63%	9 Missing ⚠️

❌ Your patch check has failed because the patch coverage (52.63%) is below the target coverage (100.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #912      +/-   ##
==========================================
- Coverage   97.35%   97.32%   -0.03%     
==========================================
  Files          50       50              
  Lines       20917    21114     +197     
  Branches      639      639              
==========================================
+ Hits        20363    20549     +186     
- Misses        489      500      +11     
  Partials       65       65              

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