Alternative for set_block_type

I've never been entirely crazy about the name for the `set_block_type` method (though it could just be me).  I was instead thinking something along the lines of `set_array_storage`.  This has two advantages: The first is that the name makes clear that this associates some property with a specific *array*.  The other is that the nomenclature is confusing--in ASDF parlance a "block" is a specific physical storage format, but arrays don't *have* to be in a block (`set_block_type` for example allows setting an inline array).  The the future we might support other types of storage as well (maybe?).