Add statistics test

Additional tests for min, max, mean and stddev ops.  The tests all follow the same pattern.

1.  Generate a random test image.
2.  Get a reference to the raw data pointer.
3.  Calculate the statistic by directly using the raw data.
4.  Calculate the statistic by calling the op.
5.  Assert that the two values are the same.
Add generateFloatArrayTestImg

Add a new function to make a floating point test image.  The image is populated with random values.
Max and min bug fix

Changed MinRealType.java and MaxRealType.java test to use copies of the 'it.next()' container.
MeanRealType: bug fix

Mean was previously calculated as size/sum.  Changed to calculate sum/size.
Modify mean test

The mean test previously asserted that the mean of a random buffer was 15.625.  The mean op was subsequently changed to return "sum/size" (previously the op had returned "size/sum"). Thus the test needed to be changed to assert that the mean of the buffer is 1/15.625.

An additional simple "sanity check" was also added that works as follows:

1.  Populate all pixels of an image with a constant value of 100.0.
2.  Calculate the mean using the op.
3.  Assert that the mean is equal to the constant value of 100.0.