* add TODO on bdryconds not being implemented for AV 2d

* fixup dg_rhs in special case when one flux component is nonzero

* cleanup initialdata

* make EulerEq2d run with Kelvin Helmholtz test (not stable yet)
also substitute all max with absmax in equations.jl

* export broadcast_bdry_2!

* clean up EulerEq/equation.jl; remove local LLF definitions

* move out numerical utils in separate file