add standard shape/dims/coords attributes

[delgadom]

would be great to improve our xarray feel, with features like

>>> cs = pjs.ComponentSet(a=range(5), b=list('abcd'))
>>> cs.a
<Component [0, 1, 2, 3, 4, 5]>
>>> cs.dims
('a', 'b')
>>> cs.shape
(5, 4)