ENH: refactor classifier handlers for application

.../Applications/AppClassification/include/otbTrainBoost.txx

...ations/AppClassification/include/otbTrainDecisionTree.txx

...AppClassification/include/otbTrainGradientBoostedTree.txx

...es/Applications/AppClassification/include/otbTrainKNN.txx

...Applications/AppClassification/include/otbTrainLibSVM.txx

...tions/AppClassification/include/otbTrainNeuralNetwork.txx

...cations/AppClassification/include/otbTrainNormalBayes.txx

...tions/AppClassification/include/otbTrainRandomForests.txx

...es/Applications/AppClassification/include/otbTrainSVM.txx