Update atom i&j partners correctly

[Ruibin-Liu]

When deleting and adding a bond between atom i and j, the previous actions:

delete!(atom_i.partners, atom_j.i)
delete!(atom_j.partners, atom_j.i)

and

 push!(atom_i.partners, atom_j.i)
 push!(atom_j.partners, atom_j.i)

would get atom i's partners correctly, but only delete/add itself from atom j's partners. Instead, we should change as below:

delete!(atom_i.partners, atom_j.i)
delete!(atom_j.partners, atom_i.i)

and

 push!(atom_i.partners, atom_j.i)
 push!(atom_j.partners, atom_i.i)

[jgreener64]

You are correct, thanks.