remove interpolation selection from cpp code

bfps/cpp/Lagrange_polys.cpp

bfps/cpp/Lagrange_polys.hpp

bfps/cpp/slab_field_particles.cpp

bfps/cpp/slab_field_particles.hpp

bfps/cpp/spline_n1.cpp

bfps/cpp/spline_n2.cpp

bfps/cpp/spline_n3.cpp

bfps/cpp/spline_n4.cpp

bfps/cpp/spline_n5.cpp

bfps/cpp/spline_n6.cpp

bfps/cpp/tracers.cpp

bfps/cpp/tracers.hpp

setup.py