Added 19 unit tests of AttrTree and ViewRef
Added 19 unit tests of AttrTree and ViewRef

basnijholt/holoviews,vascotenner/holoviews,vascotenner/holoviews,mjabri/holoviews,mjabri/holoviews,basnijholt/holoviews,ioam/holoviews,ioam/holoviews,mjabri/holoviews,ioam/holoviews,vascotenner/holoviews,basnijholt/holoviews