Should basis have __getitem__?

[billbrod]

With #289 , basis objects have __len__ and __iter__ methods. Should they also have __getitem__? This is only useful for AdditiveBasis, and would rely on storing something like np.fromiter(self.__iter__(), np.object_) (using arrays allows us to work with boolean indexing).

But the difficulty comes about with a tree structure, e.g.,:

a = Add(
    Add(b1, b2),
    Add(b3, b4),
)

Should a[1:] return

a = Add(
    b1,
    Add(b3, b4),
)

or

a = Add(
    Add(b2, b3),
    b4,
)

The first one respects the existing structure of a, but the second is much easier to create (just map sum over the remaining basis objects). The two should function identically, regardless.

Could get the first if we keep track of the tree structure, as used by the label.

(the atomic basis and multiplicative basis all can only accept 0 as just return themselves)