* support empty groups in reductions

* simplify conditional

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

* don't set a default initial value in nodal_min/nodal_max

* test with and without initial value

* Revert "don't set a default initial value in nodal_min/nodal_max"

This reverts commit 771fb7bab3a18e64248fe7725f53c77a48ef6ae6.

decided against this; not setting an initial value means that initial
value always needs to be passed in multi-volume case

* add test case for empty + no initial value

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

* Support array containers in core grudge DG ops

* Convert wave example to use array containers

* Update projection routine to support array containers

* Convert mpi communication

* Use DOFArray-based traversal routines

* Containerize distributed (eager) MPI communication

* Use array container in wave-op-mpi example

* Clean up wave-op-mpi demo

* Use non-recursive array traversal

* Use map_array_container in trace pair exchange

* Remove unused imports

* Remove sketchy assert; shape differs when evaluating fluxes vs storing state

* Refactor containerization routines

* Fix suppressed logging error

* Fix forward flattening map

* Fix type check in projection routine

* dofdesc-itize descriptors first in projection

* Make flattening methods private and provide short docs

* Clean up flatten/unflatten functions and document

* Carefully track ordering of data in flattened array

* Use container flatten/unflatten functions from arraycontext

* Containerize elementwise reductions and add unit tests

* Containerize nodal reductions and add unit tests

* Update documentation to reflect container support

* Catch scalars in op.norm

* Remove flat_norm and use op.norm instead

* Separate initialization and completion of send/recv

* Fix documentation in operator module

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

* Rename boundary communicator data/attributes

* Comment on nonblocking MPI communication process in communicator class

* Clean up comments related to rank-communication for trace pairs

* Clean up/clarify documentation related to array containers

* Update reduction unit tests

* Documentation updates

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

* Fix comment formatting

* Containerize grad/div functions

* Add checks for empty arrays in grad/div operators

* Fix gradient op for vector-valued components

* Correct div helper for handling empty and nested obj arrays

* Clear up documentation and add clarifying notes

* More shared code in array container support (#191)

* Refactor grad/weak grad to use the same helper, simplify div empty handling

* Factor out some redundant code in elementwise reductions

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