add _rate_prototype function for getting the type of du from the type of u and t

Author: oscarddssmith

ext/DiffEqBaseUnitfulExt.jl

@@ -30,4 +30,5 @@ end
     real(abs2(x) / oneunit(x) * oneunit(x))
 end
 
+_rate_prototype(u, t, onet) = u / unit(t)
 end

src/utils.jl

@@ -61,3 +61,5 @@ function default_logger(logger)
 
     LoggingExtras.TeeLogger(logger1, logger2)
 end
+
+_rate_prototype(u, t::T, onet::T) where {T} = u / oneunit(t)