public boolean iterate(Double o) {
      if (o != null) {
        state.mSum += o;
        state.mCount++;
      }
      return true;
    }
Iterate through one row of original data.

The number and type of arguments need to the same as we call this UDAF
from Hive command line.

This function should always return true.
contrib/src/java/org/apache/hadoop/hive/contrib/udaf/example/UDAFExampleAvg.java