Fix AIREBO missing derivative in bondorderLJ

This change replaces the bondorderLJ() function with code provided
by Github user CF17, which is based on the bondorder() code.
It could be fixed with a shorter patch [1], but layering fix upon
fix seems to be unwise in this case.
While the code at this point departs from following the Fortran
code closely, the reason is that the bug is present in the Fortran
code as well.
Instead, the new code follows closely the bondorder() code that
already exists, which should be easier to maintain in the future.
This patch makes the two functions consistent with each other,
and makes outside contributions easier.
Since it uses a different approach to compute its value, some
explanation of that reasoning has been added on top.

1: https://github.com/v0i0/lammps/commit/e8c5c662b28914de6b77e0ca49010eddfe1423e9