FactorMap finds column maximums incorrectly on large data sets

FactorMap's fit method does not properly find the maximum of the column.

Likely due to an impropper allreduceBlock here
https://github.com/apache/mahout/blob/master/math-scala/src/main/scala/org/apache/mahout/math/algorithms/preprocessing/AsFactor.scala#L40

Also, factorMap in this instance might be more appropriately named "factorMax"