Added missing import in core.util
Added missing import in core.util

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