weighted_variance_impl divides by zero if the first weight is zero

[jukofyork]

Hi,

lazy_weighted_variance_impl works fine if the first weight is zero, but weighted_variance_impl divides by zero:

        if(cnt > 1)
        {
            extractor<MeanFeature> const some_mean = {};

            result_type tmp = args[parameter::keyword<Tag>::get()] - some_mean(args);

            this->weighted_variance =
                numeric::fdiv(this->weighted_variance * (sum_of_weights(args) - args[weight]), sum_of_weights(args))
              + numeric::fdiv(tmp * tmp * args[weight], sum_of_weights(args) - args[weight] );
        }

It looks like "cnt > 1" should instead check "sum_of_weights(args)>0", but I'm not sure as there is some mention of this in cosurgi's answer here:

https://stackoverflow.com/questions/16761965/weighted-variance-and-weighted-standard-deviation-in-c

and this change may end up breaking existing code that takes this into account...

If this is left as is, perhaps the documentation needs updating as currently it just says:

"Note that the sample variance is not defined for n<=1".

https://www.boost.org/doc/libs/1_73_0/doc/html/boost/accumulators/impl/weighted_variance_impl.html

Juk