* Handle use-before-assign warnings from pylint

* fix remaining flake8, pylint errors

---------

Co-authored-by: Matthias Diener <mdiener@illinois.edu>

* Add Euler module

* Add simple vortex example

* Document Euler operator and add pulse experiment

* Test euler examples in lazy mode

* Make rk4_step importable

* Remove copypasta from example headers

* Make vortex initial condition importable

* Add Euler convergence test to test suite

* Rename: EulerContainer -> EulerField

* Clean up vortex initial condition

* Add type annotations and rename array container

* Clean up helper functions

* Add Euler module to the docs

* 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>

Co-authored-by: Alex Fikl <alexfikl@gmail.com>

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