Return non-geometric factors for all groups (#121)

* 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>