added average L2 error
added average L2 error

giorgiobornia/femus,FeMTTU/femus,FeMTTU/femus,eaulisa/MyFEMuS,saikanth88/femus,FeMTTU/femus,FeMTTU/femus,saikanth88/femus,eaulisa/MyFEMuS,giorgiobornia/femus,saikanth88/femus,eaulisa/MyFEMuS,saikanth88/femus,giorgiobornia/femus,giorgiobornia/femus