Write a function Hbonds(anum, aname, resno, coords) that looks for pairs with a Nitrogen (second letter of the name is 'N) and Oxygen (second letter is 'O') atoms whose distance is between 2.6 and 3.2 angstroms, inclusive. Such a pair is deemed to form a hydrogen bond if the residue sequence numbers of these atoms differ by at least 2. Return a list containing the pairs of atom numbers for hydrogen bonding pairs.