Saving alt. def. of grid_index just in case

This version still has a bug because the energy grid index and the cross section
grid index are not the same when clustering is used. cross_section.F90 assumes
they are the same. I will fix that next.
Differentiated between energy grid and xs grid

The grid_index of a nuclide now always holds the energy grid index.
If no clustering is used, the same index is used for xs and energy for
a given nuclide. If clustering is used, the mapping depends on the particle
energy and may require a lookup in the codebook, which maps energy to cluster.