Determining the count of active vertices

git-svn-id: https://svn.apache.org/repos/asf/hama/trunk@1498066 13f79535-47bb-0310-9956-ffa450edef68
Merge remote-tracking branch 'upstream/trunk' into sssp
HAMA-770: add generial methods to NeuralNetwork
HAMA-770: add identity function
HAMA-770: add readFields and write for LinearRegression