Add Ralston4 explicit Runge–Kutta method to OrdinaryDiffEqLowOrderRK #66

Job	Run time
detect-changes	21s
test (DelayDiffEq_Integrators, lts)	16m 21s
test (DelayDiffEq_Interface, 1.11)	6m 58s
test (DelayDiffEq_Integrators, pre)	15m 29s
test (DelayDiffEq_Integrators, 1)	16m 3s
test (OrdinaryDiffEqAdamsBashforthMoulton_QA, 1)	10m 38s
test (DelayDiffEq_Interface, lts)	7m 55s
test (OrdinaryDiffEqAdamsBashforthMoulton, 1.11)	9m 38s
test (OrdinaryDiffEqLowOrderRK, lts)	6m 59s
test (DelayDiffEq_Regression, lts)	2m 53s
test (DelayDiffEq_Integrators, 1.11)	13m 49s
test (OrdinaryDiffEqLowOrderRK, 1.11)	6m 33s
test (DelayDiffEq_Regression, 1)	7m 27s
test (DelayDiffEq_Regression, 1.11)	11m 7s
test (OrdinaryDiffEqLowOrderRK_QA, 1)	13m 16s
test (OrdinaryDiffEqLowOrderRK, 1)	8m 51s
test (OrdinaryDiffEqAdamsBashforthMoulton, lts)	9m 38s
test (OrdinaryDiffEqLowOrderRK, pre)	22m 14s
test (DelayDiffEq_Interface, 1)	13m 48s
test (DelayDiffEq_Interface, pre)	14m 39s
test (DelayDiffEq_Regression, pre)	10m 5s
test (DelayDiffEq_QA, 1)	8m 25s
test (OrdinaryDiffEqAdamsBashforthMoulton, pre)	11m 31s
test (OrdinaryDiffEqAdamsBashforthMoulton, 1)	12m 4s
	4h 16m 42s

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