FeatureSet python api (#1243)
FeatureSet python api (#1243)

* FeatureSet python api

* add unit test

* fix unit test

* meet code review

* fix style

* fix scala style check

* fix python style check

* add unit test

* change ut

intel-analytics/BigDL,intel-analytics/BigDL,yangw1234/BigDL,yangw1234/BigDL,yangw1234/BigDL,intel-analytics/BigDL,yangw1234/BigDL,intel-analytics/BigDL