Added Store.id_offset method to compute id offsets across backends
Added Store.id_offset method to compute id offsets across backends

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