Added additional logic to only group and sum finite values. This in effect, ignores NaN.  And, I think removes the need for the keyword nan_as_zero, which replaced nan's with 0, but then averaged these 0 values into the mean and std.dev of the output.
added comment
Removed the ';' to comment out the line that implements nan_as_zero for NaN -> 0 .
Commented out the nan_as_zero keyword in the call to group_and_sum
For test purposes: An output file on sunspot blocking (excluding missing areas in the averages), and plots of comparisons to Judith's output.