Simplify pickle-serialization of DataPreprocessor
Created by: gkirgizov
Main change is that PipelineStructureAnalysis isn't stored in DataPreprocessor. This simplifes pickling/unpickling Pipeline with preprocessor because now it doesn't store nx.DiGraph with Nodes deep internally in preprocessor.structure_analysis.
It's required for #750, because in that PR there're problems with pickle due to changes in Graph implementation that tried to unpickle old variants of Nodes stored accidentally deep inside preprocessor.