Changeset 835 for trunk/sources/HeuristicLab.GP/Recombination/SizeConstrictedGPCrossoverBase.cs

• added another abstract base class for GP crossover operators with maxsize and maxheight constraints
• changed StandardCrossOver to inherit from SizeConstrictedGPCrossoverBase
• changed SizeFairCrossOver to inherit from SizeConstrictedGPCrossoverBase
• generally improved code of SizeFairCrossOver
• changed LangdonHomologousCrossOver to inherit from SizeFairCrossOver and implemented only the method to finally select branches.

#393 (Refactor GP crossover operators to extract common code into the abstract base class GPCrossoverBase)