* 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>

The problem is that in 1D faces are 0D and so carry exactly no
useful information about which side of the element they may be
on.

To hack around that, this commit introduces a hacky operator
that hacks it. Less ambiguously, we just use the connection to
figure out which face we're on on set the normal to `+1` or `-1`
based on that