Feature/time integration
Introducing RK2_SSP and RK3_SSP time-stepping schemes and adjusting the names of some previous schemes (with backwards compatibility) as follows:
"Midpoint" scheme now called "RungeKutta2" (old name still works)
"RungeKutta2_ImprovedEuler" scheme now called "RungeKutta2_SSP" (old name still works)
"RungeKutta2_ModifiedEuler" scheme replaced by "RungeKutta3_SSP" (old scheme was not working)
"ClassicalRungeKutta4" scheme now called "RungeKutta4" (old name still works)
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