Added Columns.__setstate__ method for backwards compatibility
Added Columns.__setstate__ method for backwards compatibility

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