Correct iterativeMean implementation for images

Otherwise get:

```
Traceback (most recent call last):
  File "/home/benjamin/robust_image_source_identification_on_modern_smartphones/datasets/raise/./extract_noise.py", line 148, in <module>
    treatImage(imageFileName, color = color)
  File "/home/benjamin/robust_image_source_identification_on_modern_smartphones/datasets/raise/./extract_noise.py", line 114, in treatImage
    estimatedPrnuIterativeMean.add(imageNoiseNpArray)
  File "/home/benjamin/robust_image_source_identification_on_modern_smartphones/datasets/raise/utils.py", line 34, in add
    self.mean = ((self.mean * self.numberOfElementsInMean) + element) / (self.numberOfElementsInMean + 1)
                  ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
TypeError: unsupported operand type(s) for *: 'NoneType' and 'int'
```

datasets/raise/utils.py

@@ -31,5 +31,8 @@ class iterativeMean:
     numberOfElementsInMean = 0
 
     def add(self, element):
-        self.mean = ((self.mean * self.numberOfElementsInMean) + element) / (self.numberOfElementsInMean + 1)
-        self.numberOfElementsInMean += 1
+        if self.mean is None:
+            self.mean = element
+        else:
+            self.mean = ((self.mean * self.numberOfElementsInMean) + element) / (self.numberOfElementsInMean + 1)
+        self.numberOfElementsInMean += 1