added RK4 and it runs
added RK4 and it runs

saikanth88/femus,saikanth88/femus,giorgiobornia/femus,FeMTTU/femus,giorgiobornia/femus,giorgiobornia/femus,saikanth88/femus,saikanth88/femus,giorgiobornia/femus,FeMTTU/femus,FeMTTU/femus,FeMTTU/femus