Calculate the standard error of the mean

## See also

Java code can be found here: https://github.com/beast-dev/beast-mcmc/blob/800817772033c13061f026226e41128d21fd14f3/src/dr/inference/trace/TraceCorrelation.java#L159 # nolint URLs can be long

Calculate the standard error of the mean

Java code can be found here: https://github.com/beast-dev/beast-mcmc/blob/800817772033c13061f026226e41128d21fd14f3/src/dr/inference/trace/TraceCorrelation.java#L159 # nolint URLs can be long