Fixed incomplete implementation of __add__ method for GridLayout
Fixed incomplete implementation of __add__ method for GridLayout

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