* 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>

* Update function: dt_non_geometric_factor -> dt_non_geometric_factors

* Add timestep estimation helper function

* Add clarifying remark about timestep estimation

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

* Make wavespeed argument option

* Drop dt estimator; provide characteristic lengthscales

* Expand and update documentation

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

* Allow HyperbolicOperator.max_characteristic_velocity to return local data

* Add warning to dt_geometric_factors about non-volume discretizations

* Add fudge factor to surface advection time step

* Time step estimation: clean up freeze/thaw semantics

* Wave models: clean up freeze/thaw

* Variable coefficient wave model: do not nodal-reduce characteristic velocity

* Add missing freeze(...) for dim=1 case

* Add missing thaw in dt utils unit test

* Fix variable coefficient wave example

* Memoize characteristic lengths rather than geometric factors

* Use actx from wave speed DOFArray in variable coefficient operator

* Pass array context to estimate_rk4_timestep

* Ensure application actx is passed through to timestep estimation

* Update Operator method interfaces for timestepping

* Always return thawed characteristic lengthscales

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

* Make nodal reductions mpi-aware

* Add clarifying remark for nodal reductions

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

* Use timestep estimation in mpi wave test

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

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

For 687a28bf: Drop StrongWaveOperator, fix WeakWaveOperator, retool dagrt-fusion to use that directly

* uses logger instead of print
* uses the provided `ctx_factory` instead of `create_some_context`