[WIP] added islice + flatten for iterable datapipes

[dbish]

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Fixes #656

Changes

• added new iterdatapipe called islice for selecting a slice of an iterable item returned (ex: list, tuple, dict)
• added indexing as part of this islice functionality, allowing a list of indexes (or keys) to also be used in place of a slice for selection
• also added flatten iterdatapipe to allow for an item in an iterable to be flattened one level if an index or list of indices are provided
• added default for flatten (no args) that yields all items for all iterables in the provided datapipe chained together