issues: 289837692

When I am writing unit tests for routines that involve DataArray objects many lines of code are devoted to creating mock objects. Here is an example of a unit test I recently wrote to test some code which computes the fluid derivative of a field given the velocity. ```python def test_material_derivative():

dims = ['x', 'y', 'z', 'time']
coords = {dim: np.arange(10) for dim in dims}
shape = [coords[dim].shape[0] for dim in coords]

f = xr.Dataset({'f': (dims, np.ones(shape))}, coords=coords)
f = f.f

one = 0 *f +1
zero = 0*f


md = material_derivative(zero, one, zero, f.x + 0*f)
np.testing.assert_array_almost_equal(md.values, 0)

md = material_derivative(one, zero, zero, f.x + 0*f)
np.testing.assert_array_almost_equal(md.isel(x=slice(1,-1)).values,
                                     one.isel(x=slice(1,-1)).values)

md = material_derivative(zero, one, zero, f.y + 0*f)
np.testing.assert_array_almost_equal(md.isel(y=slice(1,-1)).values,
                                     one.isel(y=slice(1,-1)).values)

md = material_derivative(zero, zero, one, f.z + 0*f)
np.testing.assert_array_almost_equal(md.isel(z=slice(1,-1)).values,
                                     one.isel(z=slice(1,-1)).values)

```

As you can see, I devote many lines to initializing a 4D data array of all ones, where all the coordinates are np.arange(10) objects. It isn't too hard to do this once, but it gets pretty annoying to do many times, especially when I forget how the DataArray and Dataset constructors work. Now, I can do something like xr.DataArray(np.ones(...)), but I would still have to initialize the coordinates if I use them.

In any case, having some sort of functions like xr.ones, xr.zeros, and xr.rand which initialize the coordinates and data would be very nice.