Update SISSORegressor to set up for SISSOClassifier

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/main.cpp

src/python/bindings_docstring_keyed.cpp

src/python/bindings_docstring_keyed.hpp

src/python/descriptor_identifier/SISSORegressor.cpp

src/python/descriptor_identifier/SISSO_DI.cpp