Reduce allocations for matrix differential equations function

[c42f]

@Egwene_al_Vere

I guess for now, the solution is to use static arrays by Chris_Foster

Well, I feel like I should point out that StaticArrays is the work of a lot of people: Contributors to JuliaArrays/StaticArrays.jl · GitHub. Andy Ferris is the main author and maintainer; I help him out now and then as do several other people.