Weights adding to 0 not an error

[Carlos Parada]

julia> p = pweights(zeros(10))
10-element ProbabilityWeights{Float64, Float64, Vector{Float64}}:
 0.0
 0.0
 0.0
 0.0
 0.0
 0.0
 0.0
 0.0
 0.0
 0.0

Weights should probably throw an error if the sum is 0.