Regressors now work with LossFunction()

src/descriptor_identifier/SISSO_DI/SISSOClassifier.cpp

src/descriptor_identifier/SISSO_DI/SISSOLogRegressor.cpp

src/descriptor_identifier/SISSO_DI/SISSOLogRegressor.hpp

src/descriptor_identifier/SISSO_DI/SISSORegressor.cpp

src/descriptor_identifier/SISSO_DI/SISSORegressor.hpp

src/descriptor_identifier/SISSO_DI/SISSO_DI.cpp

src/descriptor_identifier/SISSO_DI/SISSO_DI.hpp

src/feature_creation/feature_space/FeatureSpace.cpp

src/loss_function/LossFunction.cpp

src/loss_function/LossFunction.hpp

src/loss_function/LossFunctionLogPearsonRMSE.cpp

src/loss_function/LossFunctionLogPearsonRMSE.hpp

src/loss_function/LossFunctionPearsonRMSE.cpp

src/loss_function/LossFunctionPearsonRMSE.hpp

src/loss_function/utils.cpp

src/python/descriptor_identifier/SISSOClassifier.cpp

src/python/descriptor_identifier/SISSOLogRegressor.cpp

src/python/descriptor_identifier/SISSORegressor.cpp

src/python/descriptor_identifier/SISSO_DI.cpp

src/utils/project.cpp