Create googletests for C++ unit tests

CMakeLists.txt

src/descriptor_identifier/Model/Model.hpp

src/descriptor_identifier/Model/ModelClassifier.cpp

src/descriptor_identifier/Model/ModelLogRegressor.cpp

src/descriptor_identifier/Model/ModelLogRegressor.hpp

src/descriptor_identifier/Model/ModelRegressor.cpp

src/descriptor_identifier/Model/ModelRegressor.hpp

...rator_nodes/allowed_operator_nodes/abs/absolute_value.cpp

...re_creation/node/value_storage/nodes_value_containers.cpp

src/feature_creation/units/Unit.cpp

src/feature_creation/units/Unit.hpp

src/python/bindings_docstring_keyed.cpp

src/python/postprocess/utils.py

tests/googletest/CMakeLists.txt

.../descriptor_identification/model/test_model_classifier.cc

...scriptor_identification/model/test_model_log_regressor.cc

...t/descriptor_identification/model/test_model_regressor.cc

...r_identification/sisso_regressor/test_sisso_classifier.cc

...dentification/sisso_regressor/test_sisso_log_regressor.cc

...or_identification/sisso_regressor/test_sisso_regressor.cc