Merge branch 'master' of gitlab.mpcdf.mpg.de:tpurcell/cpp_sisso into joss

src/feature_creation/feature_space/FeatureSpace.hpp

src/feature_creation/node/utils.cpp

src/feature_creation/node/utils.hpp

src/python/py_binding_cpp_def/bindings_docstring_keyed.cpp

...thon/py_binding_cpp_def/feature_creation/FeatureSpace.cpp

...python/py_binding_cpp_def/feature_creation/node_utils.cpp

...python/py_binding_cpp_def/feature_creation/node_utils.hpp

tests/googletest/feature_creation/utils/test_utils.cc

...feature_creation/test_feature_space/feature_space/phi.txt