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 sure, but I don't see why not. Do you see any advantage putting it inside? For some reason, it looks to me cleaner this way, although I always prefer to have methods implemented in cpp files.