Move arg_max/max_abs/arg_max_abs/arg_min from SGVector to CMath

I noticed that max_abs and arg_max_abs are never used anywhere in the code. Should I remove them entirely from the library?
@iglesias Here I had to explicitly call `CMath::arg_max<float64_t> ()` instead of just `CMath:::arg_max()`, otherwise it was giving an error. Why is that?