turn off -march=native by default

valgrind reports an unrecognized instruction when compiled with
-march=native
add example potential using fortran and iso_c_bindings

fortran with iso_c_bindings is called from python through cython

there appears to be something wrong with the example c_potential not
sure what it is
fix the example c potential

It was completely broken, I don't know how it got originally submitted
in such a broken state