initialize self.last_occupancy to very small value

self.last_occupancy was initialized as -1 and used later to help decide which disordered atom will be chosen as representative. If the first atom in DisorderedAtom already has -1 occupancy, it will not be chosen as it does not satisfy the comparison. NoneType error will be thrown if, say, get_fullatom() function is invoked.
Since occupancy value occupies col 55-60 (6 digits). It can never get smaller than -999999 and this issue will never appear again.
Thank Hong Zhu for PBD tweak

This was pull request #445 to close issue #441 (parsing PDB 3NH3)