Add an add() method to pyspark accumulators.

Add a regular method for adding a term to accumulators in
pyspark. Currently if you have a non-global accumulator, adding to it
is awkward. The += operator can't be used for non-global accumulators
captured via closure because it's involves an assignment. The only way
to do it is using __iadd__ directly.

Adding this method lets you write code like this:

def main():
    sc = SparkContext()
    accum = sc.accumulator(0)

    rdd = sc.parallelize([1,2,3])
    def f(x):
        accum.add(x)
    rdd.foreach(f)
    print accum.value

where using accum += x instead would have caused UnboundLocalError
exceptions in workers. Currently it would have to be written as
accum.__iadd__(x).

python/pyspark/accumulators.py

@@ -42,6 +42,13 @@
 >>> a.value
 13
 
+>>> b = sc.accumulator(0)
+>>> def g(x):
+...     b.add(x)
+>>> rdd.foreach(g)
+>>> b.value
+6
+
 >>> from pyspark.accumulators import AccumulatorParam
 >>> class VectorAccumulatorParam(AccumulatorParam):
 ...     def zero(self, value):
@@ -139,9 +146,13 @@ def value(self, value):
             raise Exception("Accumulator.value cannot be accessed inside tasks")
         self._value = value
 
+    def add(self, term):
+        """Adds a term to this accumulator's value"""
+        self._value = self.accum_param.addInPlace(self._value, term)
+
     def __iadd__(self, term):
         """The += operator; adds a term to this accumulator's value"""
-        self._value = self.accum_param.addInPlace(self._value, term)
+        self.add(term)
         return self
 
     def __str__(self):