ProductSpace.__getitem__ should return a Productspace

[adler-j]

As noted in implementing #53 ProductSpace.__getitem__ currently returns a list if indices is a slice:

return self.spaces[indices]

In the case where indices is a slice, ProductSpace.__getitem__ should do:

return ProductSpace(self.spaces[indices])

to be sure, this should fail if the space has a custom norm (since this does not transfer trivially).

if indices is a int, we should simply

return self.spaces[indices]