GetAttr cleanup and doc

[pete88b]

• added docs and tests to help people understand GetAttr#_xtra https://forums.fast.ai/t/fastai-v2-chat/53518/108
• added _component_attr_filter so we use the same logic in _dir and __getattr__
  • now _dir only returns attributes that will be accessible
  • i.e. if _xtra = ['lower'], you can't call .upper(), so I don't think it makes sense to have upper returned by __dir__
  • to do this, we need to reference _xtra from _dir - which means _xtra can't call _dir - or we'll recurse forever
• changed self._dir() if self._xtra is None else self._dir() to self._dir()
  • i'm guessing this was/should have been self._dir() if self._xtra is None else self._xtra but as _dir uses _xtra in it's filter, we don't need a special condition for this now

I've not changed this but L#_xtra seems to be unused - might it make sense to remove this?

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[jph00]

Many thanks!