@Override
    public List<TensorFunction<NAMETYPE>> arguments() { return List.of(argument); }
The <i>expand</i> tensor function returns a tensor with a new dimension of
size 1 is added, equivalent to "tensor * tensor(dim_name[1])(1)".

@author lesters
vespajlib/src/main/java/com/yahoo/tensor/functions/Expand.java