Merge IsGraphvizGraph, IsGraphvizDigraph and IsGraphvizContext into a single type

[reiniscirpons]

This PR unifies IsGraphvizGraph, IsGraphvizDigraph and IsGraphvizContext into a single type IsGraphvizGraph.
We could recover these types by re-adding them as properties, but since all of these types share the same interface, there is no need to have different constructors and filters for them.

Fixes #115 .

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.43%. Comparing base (5169cac) to head (fcdf8ec).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #121      +/-   ##
==========================================
- Coverage   94.90%   94.43%   -0.47%     
==========================================
  Files          12       12              
  Lines        1255     1223      -32     
==========================================
- Hits         1191     1155      -36     
- Misses         64       68       +4     

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[james-d-mitchell]

Is it still possible to disambiguate contexts from graphs and digraphs? This is important in the changes I'm making.

[reiniscirpons]

Is it still possible to disambiguate contexts from graphs and digraphs? This is important in the changes I'm making.

Oui, g!.IsContext is true if and only if something is a context. I could add a wrapper IsGraphvizContext property which just returns this parameter if that would help

[james-d-mitchell]

Very good, I'll try to take a look today

[james-d-mitchell]

Isn't this just poor man's properties? I.e. shouldn't we just define IsGraphvizContext as a property of objects in the category IsGraphvizGraph, and likewise IsGraphvizDigraph? This is equivalent to what you're doing here in many ways, and is more "official". O/w I approve these changes.

[james-d-mitchell]

Please change !.IsContext etc to properties of IsGraphvizGraph, then I'm happy to merge this!

[james-d-mitchell]

Turned out it wasn't possible to use properties, so I re-added IsGraphvizDigraph and IsGraphvizContext. There are some good changes in this PR, but I'm not sure the one that renamed IsGraphvizGraphDigraphOrContext to IsGraphvizGraph is really the best idea.

[james-d-mitchell]

In discussion with @reiniscirpons we decided to revert IsGraphvizGraph in this PR to IsGraphvizGraphDigraphOrContext but to keep the other refactoring/simplification code in this PR.

[james-d-mitchell]

The rebase was non-trivial, it looks like I haven't completed it properly, please don't merge until I can come back to it.