Add iterativeMean to utils

Verified with:

```py
myIterativeMean = iterativeMean()
print(myIterativeMean.mean)
myIterativeMean.addElement(2)
print(myIterativeMean.mean)
myIterativeMean.addElement(3)
print(myIterativeMean.mean)
```

Note that cannot simplify as follows in the general case of using images as
`element`s for instance:

```diff
diff --git a/datasets/raise/utils.py b/datasets/raise/utils.py
index fba5523..a116edb 100644
--- a/datasets/raise/utils.py
+++ b/datasets/raise/utils.py
@@ -27,12 +27,9 @@ def denoise(imageNpArray, denoiserName):
     return imageDenoisedNpArray

 class iterativeMean():
-    mean = None
+    mean = 0
     numberOfElementsInMean = 0

     def addElement(self, element):
-        if self.mean is None:
-            self.mean = element
-        else:
-            self.mean = ((self.mean * self.numberOfElementsInMean) + element) / (self.numberOfElementsInMean + 1)
+        self.mean = ((self.mean * self.numberOfElementsInMean) + element) / (self.numberOfElementsInMean + 1)
         self.numberOfElementsInMean += 1
\ No newline at end of file
```

datasets/raise/utils.py

@@ -24,4 +24,12 @@ def denoise(imageNpArray, denoiserName):
             imageDenoisedNpArray = skImageRestorationDenoise(imageNpArray, sigma_color=0.05, sigma_spatial=15)
         case 'tv_chambolle':
             imageDenoisedNpArray = skImageRestorationDenoise(imageNpArray, weight=0.2)
-    return imageDenoisedNpArray
+    return imageDenoisedNpArray
+
+class iterativeMean():
+    mean = 0
+    numberOfElementsInMean = 0
+
+    def addElement(self, element):
+        self.mean = ((self.mean * self.numberOfElementsInMean) + element) / (self.numberOfElementsInMean + 1)
+        self.numberOfElementsInMean += 1