[python package] added get_ref_chain function to Dataset object, used it in train() to… (#745)
[python package] added get_ref_chain function to Dataset object, used it in train() to… (#745)

* added get_ref_chain function to Dataset object, used it in train() to compare if training and validation have common reference ancestors

* moved check of common ancestor reference from train() in engine.py to set_reference() in Dataset

* moved check of common ancestor reference from train() in engine.py to set_reference() in Dataset

* removed check for handle = None

microsoft/LightGBM,wxchan/LightGBM,Laurae2/LightGBM,olofer/LightGBM,henry0312/LightGBM,Laurae2/LightGBM,microsoft/LightGBM,Laurae2/LightGBM,microsoft/LightGBM,Allardvm/LightGBM,Laurae2/LightGBM,Laurae2/LightGBM,Allardvm/LightGBM,henry0312/LightGBM,henry0312/LightGBM,henry0312/LightGBM,olofer/LightGBM,henry0312/LightGBM,microsoft/LightGBM,Allardvm/LightGBM,wxchan/LightGBM,olofer/LightGBM,Allardvm/LightGBM,wxchan/LightGBM,olofer/LightGBM,olofer/LightGBM,Allardvm/LightGBM,wxchan/LightGBM,wxchan/LightGBM,microsoft/LightGBM