* 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>

* Clone _setup_actx to permit catching leaks

* Point arraycontext at freeze-thaw-usability in req.txt

* Point requirements.txt back to main for arraycontext

* bessel function mangling -> BesselFunction: ScalarCallable

* query translation unit's default_entrypoint's args

* [drop before merge]: test with kcv3-e2

* Add transfer_requirements_git_urls from grudge to downstream projects

* Point requirements.txt for loopy back to main

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

* Add jump property to TracePair.

* Change jump --> diff

* Add simple smoke test for trace pair

* placate flake8

* Undo spurious chage to primitives.py

* Tweak TracePair docstrings

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

Retire redundant examples

* 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>

Grudge Refactoring

Update time-step utility functions

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

* Use mpi4py from conda for tests

* Drop "insecure" flag from curl in Github CI

* Drop "insecure" flag from curl in Gitlab CI

* Simplify WADG mass inverse

* Test mass inverse on warped geometry

* Always WADG all the time also for symbolic

* Fold test_nonaffine_mass_operator_inverse into test_mass_operator_inverse

Co-authored-by: Thomas Gibson <gibsonthomas1120@hotmail.com>