Support for key-value descriptors.

src/soap/linalg/bindings.cpp

src/soap/linalg/permanent/CMakeLists.txt

src/soap/linalg/permanent/permanent.cpp

src/soap/soapy/dimred.py

src/soap/soapy/kernel.py

src/soap/soapy/lamatch.py

src/soap/soapy/pca.py

src/soap/spectrum.cpp

src/soap/spectrum.hpp