Revert "Revert "Added support for a writeblock as the output selection for a query""

This reverts commit 7dee133ab4c57679e74ed60e4bfc396fbee507ee.
Reworked handling of writeblocks, since a writeblock selection is necessarily variable-relative (it depends on that variable's blocks), but the user specifies it once for the entire query. Since the query may span multiple variables (even multiple files), we must ensure that the block bounds match for the given writeblock across all participating variables, and then use this common varblock as the bounding box in place of the writeblock selection
Merge branch 'drew-wip' into adios-query