When calculate standard deviation forget to divide error.size()

[huhumeng]

Hello, i find a bug in

rpg_vikit/vikit_common/src/robust_cost.cpp

Line 84 in 10871da

return std::sqrt(var); // return standard deviation

which forget to divide the errors number to get a standard deviation. It may cause to large scale to weighted the residual, and cannot to suppress outliers.