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?
I am not 100% sure whether they are the same, but it makes sense to me. I'd say that as long as compilation continues going smoothly is fine. Anyway, as we just discussed, these methods are going to be dropped probably as they are unused.