Statistics fixes

When attempting to use the statistics ops to verify results from other ops I was writing I noted possible errors in the statistics ops.   Thus I added a new test, StatisticsTest.java.  I also added a 'generateFloatArrayTestFunction' to AbstractOpsTest.java.

To get the new test to pass I did the following 

1.  Changed MinRealType.java and MaxRealType.java test to use copies of the 'it.next()' container.
2.  Changed MeanRealType.java op calculation to 'sum/size' (it was 'size/sum'). 

I also could not get a stddev test to pass because there were multiple signatures with the same priority.  But I wasn't sure of the best way to fix this. 
Could you please fix the indentation here? We try to use tabs consistently so that everybody can choose their particular preference as to how many spaces there should be (we typically use two spaces/tab in the ImageJ2 project).