[FLINK-1901] [core] enable sample with fixed size on the whole dataset.

[FLINK-1901] [core] add more comments for RandomSamplerTest.

[FLINK-1901] [core] refactor PoissonSampler output Iterator.

[FLINK-1901] [core] move sample/sampleWithSize operator to DataSetUtils.

Adds notes for commons-math3 to LICENSE and NOTICE file

This closes #949.