Fixed a bug in msmbuilder/Extras/parallel_assign/lib/remote.py regarding application of AtomIndices.dat

Generators were mistakenly screened with AtomIndices.dat, when all atoms from generators should be used. This fix sets the atom indices attribute to the 'metric' object to None when computing the generators and then restores it afterwards. I've tried this on the ala2 system and it produced correct results.