Provide a helper function to flatten a Chunk that wraps a StructArray

[ncpenke]

From the discussion in: #40.

When converting a Vec<S> into an arrow2::Chunk via try_into_arrow where S is a struct, the resulting Chunk will wrap a StructArray. We'd like to facilitate the conversion of this Chunk to another Chunk that directly wraps the fields of the StructArray.

One way this could be accomplished is by introducing a FlattenChunk trait, which defines a flatten method, that consumes a Chunk and returns a modified Chunk with the fields of a StructArray or the original Chunk if it was not wrapping a StructArray. This trait could be implemented for Chunk<A> where A: AsRef<dyn Array>.