[FEATURE REQUEST] Add archive indexing

[btjanaka]

Description

Passing in an index, such as one from get_index(), should give back the Elite at the given index. Furthermore, we should be able to do some fancy slicing, which will result in a new archive. We may start with only implementing this in GridArchive.

It would be nice if we could get feedback on whether this feature would be useful, as it may be a bit complicated to implement.