Adding: Multiple residuals fitting and leave-out CV keys

src/descriptor_identifier/Model/Model.cpp

src/descriptor_identifier/Model/Model.hpp

src/descriptor_identifier/SISSORegressor.cpp

src/descriptor_identifier/SISSORegressor.hpp

src/feature_creation/feature_space/FeatureSpace.cpp

src/feature_creation/feature_space/FeatureSpace.hpp

src/feature_creation/node/FeatureNode.cpp

src/feature_creation/node/FeatureNode.hpp

src/feature_creation/node/Node.cpp

src/feature_creation/node/Node.hpp

src/feature_creation/node/operator_nodes/OperatorNode.cpp

src/feature_creation/node/operator_nodes/OperatorNode.hpp

...ator_nodes/allowed_operator_nodes/absolute_difference.cpp

...ator_nodes/allowed_operator_nodes/absolute_difference.hpp

.../operator_nodes/allowed_operator_nodes/absolute_value.cpp

.../operator_nodes/allowed_operator_nodes/absolute_value.hpp

...eation/node/operator_nodes/allowed_operator_nodes/add.cpp

...eation/node/operator_nodes/allowed_operator_nodes/add.hpp

...eation/node/operator_nodes/allowed_operator_nodes/cos.cpp

...eation/node/operator_nodes/allowed_operator_nodes/cos.hpp