emfmm3d.m (Matlab) rename targ outputs to match other 3 kernels

[ahbarnett]

I might suggest that emfmm3d return U.Etarg, U.curlEtarg, U.divEtarg
for consistency with the "targ" suffix for all other kernels. That way when source-eval is added, it won't break user code! (will it break current user code, though, @mrachh, @askhamwhat ? )
Do you agree with the change? Thx, Alex