creating this as a placeholder for implementing the nanflag options for the moving window functions that allows them to include/exclude NaN elements.

This has been implemented in the moving function options for `fillmissing` in the statistics package, but the base functions are rather different so it may not be straightforward to port into movfun. nanflag creates a possibility that each column includes a different number of points. so efficient operation is a challenge. Many of the base functions (sum, etc, etc) have nanflag options that movfun could just pass the flag to, but the option isn't implemented for all of them, and octave having a generic movfun means that would have to support nanflag internally. This might warrant separate codepaths for built in fns