added _is_homogeneous property

[shaneding]

This PR closes #7067.
This was implemented by adding the _is_homogeneous property to DataFrame. Included are appropriate test cases.

[brandon-b-miller]

thanks for adding this :) I realize now after having requested DataFrame._is_homogenous in the original issue that this property might be better suited for our Frame class so all our column based objects get it for free.

[shaneding]

@brandon-b-miller no worries, I can move this to Frame easily.

[codecov]

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[beckernick]

@gpucibot merge

[brandon-b-miller]

great work @shaneding !