BasisExpansion: gradients I.

cxx_tests/gtest.sh

cxx_tests/src/gtest_cutoff.cpp

src/soap/angularbasis.cpp

src/soap/angularbasis.hpp

src/soap/basis.cpp

src/soap/basis.hpp

src/soap/cutoff.cpp

src/soap/cutoff.hpp

src/soap/functions.cpp

src/soap/options.cpp

src/soap/options.hpp

src/soap/radialbasis.cpp

src/soap/radialbasis.hpp

src/soap/spectrum.cpp