Add a finite volume method for ScalarEq (!75)

* add reftest fv_advection_sine

* migrate all of ScalarEq to use broadcast_*_2! methods

* add isperiodic method for meshes

* fix grid spacing computation for SpectralElement

* fixup parametric types to subtype Mesh1d/2d

* make TODO error more comprehensive

* fixup cfl usage; add abstractions for min grid spacing

* enable add mesh kwarg to Evolution setup in order to enable FV evolution

* add timestep! and rhs! for FVElement

* do away with the State(...) syntax

* add docs to dg_rhs methods

* add FV stepper

* limit dg_rhs.jl methods to Meshes with SpectralElement

* add mesh kwarg to Evolution so that we can use a spearate stepper based on mesh type

* add a scheme parameter to Mesh

* add FVElement type