Changeset 2210 for branches/GP-Refactoring-713/sources/HeuristicLab.GP.StructureIdentification/3.3/VariableFunctionTree.cs

GP Refactoring #713

• introduced a plugin for GP interfaces
• created a new interface IGeneticProgrammingModel which represents GP models in HL scopes instead of IFunctionTree
• changed interfaces IFunction and IFunctionTree
• moved some files to new directories (general housekeeping)
• changed all GP operators and engines to work with IGeneticProgrammingModels
• removed parameters TreeSize and TreeHeight in all GP operators
• changed parameter OperatorLibrary to FunctionLibrary in all GP operators