Add DataHandling.add_arrays

This adds a function for lazy people who want to write

dh = create_data_handling((20, 30))
x, y, z = dh.add_arrays('x, y(9), z')

instead of

dh = create_data_handling((20, 30))
dh.add_array('x')
dh.add_array('y', values_per_cell=9)
dh.add_array('z')

because in most cases you want to use more than one array.