Update field_statistics to get more accurate results

[oddvarlia]

Improve difference calculation between posterior and prior
Field statistics is calculated for the specified iterations from an ES-MDA update. If some realizations fails to be updated during the iterations in the MDA, there will be a difference in number of realizations in prior(iter = 0) and posterior ensemble. This will create unwanted noise when calculating
the difference between posterior and prior ensemble mean or standard deviation. This need to fixed to ensure that exactly the same set of realizations are used when calculating mean and standard deviation of both prior and posterior ensemble.

Calculate differences between posterior and prior ensemble mean of the gaussian fields used in APS
Add a difference calculation to get these difference 3D parameters as well in the generated python script when loading the results into RMS.