Add optional second output to var and std (bug #62395) * scripts/statistics/var.m: Add optional second output containing the mean used to calculate the variance. Move weight isempty check ahead of vector dimension isscalar check to avoid triggering incompatability error. Add BISTs testing second output with different calling options. Add BIST testing empty value passed as variance weight treated as zero. Add new output behavior to docstring, and update function definitions to show the primary variable. * scripts/statistics/std.m: Add passthrough for second output from var when std called with two outputs. Add BISTs testing second output with different calling options. Update docstring noting new output behavior. * etc/NEWS.8.md: Note output changes to var and std under Matlab Compatability.