gh-236: make Result container not array subclass

[ntessore]

This changes the Result class to be a container holding an array, but not be an array subclass itself. Having it be an array subclass leads to problems such as #236.

The container forwards basic operations such as result[...], result.ndim, result.shape to the array (more could be added). So users can still do

# cl_sheshe is a result of shape (3, lmax + 1)
plt.plot(ell, cl_sheshe[0])  # plot EE spectrum

The result also has an __array__() method that returns the contained array in a numpy context, so this works even if result is no longer an array itself:

# any numpy function will do
np.multiply(result, 1)

[JaimeRZP]

LGTM