Fixes two errors obtained by valgrind+verrou

 - Avoid computing the Legendre polynomial for values outside of [-1:1]
 - Fix a rounding error in which 2/3+2/3+2/3 -> 2.0000000000000004 occupations in C leads to loosing 2 electrons

Both of the errors were obtained for the test finite_system_3d/01-carbon_atom.01-psf_l0.inp