Swift Array扩展用于标准偏差

I am frequently needing to calculate mean and standard deviation for numeric arrays. So I've written a small protocol and extensions for numeric types that seems to work. I just would like feedback if there is anything wrong with how I have done this. Specifically, I am wondering if there is a better way to check if the type can be cast as a Double to avoid the need for the asDouble variable and I am frequently needing to calculate mean and s