Add DigraphByEdges variant for list of arbitrary vertex labels and list of edges

[William-Kenyon]

Fixes #778, for adding a way of constructing a Digraph with two lists: vertex_labels, and edges, where vertex labels can be more than just integers. This is basically the same as the existing 3-argument constructor (vertices, source, range), except source and range are zipped together.

Based on @wilfwilson's code snippet.

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.47%. Comparing base (f4fc1ac) to head (65aa4d9).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
gap/digraph.gi	92.59%	2 Missing ⚠️

❌ Your patch check has failed because the patch coverage (92.85%) 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     #957   +/-   ##
=======================================
  Coverage   97.47%   97.47%           
=======================================
  Files          51       53    +2     
  Lines       21301    21391   +90     
  Branches      639      639           
=======================================
+ Hits        20763    20851   +88     
- Misses        473      475    +2     
  Partials       65       65           

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[james-d-mitchell]

Thanks for the PR @William-Kenyon. I think what you've implemented should be a method for DigraphByEdges and not an overload of Digraph though. Can you please update it so that it is this, and address the other comments too? Thanks!

[james-d-mitchell]

Suggested change

	fi;
	if not (IsSubset(vertex_labels, Union(edges))) then
	elif not IsSubset(vertex_labels, Union(edges)) then

save one line, and the brackets are superfluous, so please remove.

[james-d-mitchell]

It's be useful if this line could include some details of which label is bad (position, and value), and the value of vertex_labels maybe too?