Use NamedTuples or extension thereof for solution

[glwagner]

Rather than Arrays of Arrays. A named tuple is better (it has names) and we get loop unrolling, which is probably faster than using broadcasting as done, for example, here:

FourierFlows.jl/src/timesteppers.jl

Line 211 in 1028120

function RK4substeps!(sol::AbstractArray{T}, cl, ts, eq, v, p, g, t, dt) where T<:AbstractArray