Switch to using flat Vectors for DGMulti

[jlchan]

DGMulti uses either Vector{<:SVector} or StructArray{SVector} to represent it solutions. However, OrdinaryDiffEq.jl's precompilation is designed for Vector{Float64} types. Switching DGMulti's representation to Vector and using wrap_array to convert this Vector to either

1. a Vector reinterpreted as an array of SVector
2. a StructArray{SVector} constructed from views into a contiguous vector
   should enable us to take advantage of OrdinaryDiffEq.jl's precompilation.

Additionally, Julia can only resize Vector{Float64} types, so if we want to put adaptivity into DGMulti, we probably have to do this anyways.