API:  create eelbrain.data package

- RM duplicate:  remove cell_labels(), only retain cellname()
TEST plot:  close() plots
RF ndvar.get_data():  use np.transpose()
ENH ndvar:  better time indexing accuracy through rounding
ENH as_factor():  convert any object that has .as_factor() method
ENH data-obj:  assub() to interpret sub argument
FIX/TEST plot.uv