fix: remove redundant class info for subclasses

[dandhlee]

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Previously when I simply concatenated the parent_class info to insert_class to combine the list and iterate through the combined list of the two, unfortunate things happened.

• insert_class is a pointer reference to app.env.docfx_yaml_classes[datam[CLASS]], so modifying insert_class reflects the data stored in app.env.docfx_yaml_classes and we took advantage of that below.
• When I extended insert_class with parent_class, that's where the duplication happened by echoing the parent_class data literally onto the corresponding app.env.docfx_yaml_classes[datam[CLASS]].

This is now avoided by handling them separately.

Also, when adding the subclass to its parent we only need to include its info in the reference and under children section of the yaml, so we do not need to append datam to the parent class, since this would be redundant information we echo onto the parent class.

Fixes #86

It's a good idea to open an issue first for discussion.

• Tests pass
• Appropriate changes to README are included in PR

[tbpg]

One comment, otherwise looks good.

[tbpg]

Is datum['parent'] already set to obj['uid']?

I'm pretty sure parent and children should agree.

[dandhlee]

We don't actually keep track of parent, it's only tracked in the reference after the fact.

In this case we have datam['module'] which is equivalent to datam['parent'], so I think we're good (we'll never have a subclass that's in a different module).