Import and update BlockIndexRanges from sumpy (#92)

* import and update BlockIndexRanges from sumpy

* move make_block_index to utils

* add a test for the cartesian product of matrix indices

* port uses of BlockIndexRanges to new versions

* point requirements to wip sumpy branch

* fix some small doc issues

* Point sumpy back to main in req.txt

Co-authored-by: Andreas Klöckner <inform@tiker.net>