Added pop and delitem and fixed getitem on AttrTree
Added pop and delitem and fixed getitem on AttrTree

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