bug in parallel implementation of delta_sigma

The [delta_sigma](https://halotools.readthedocs.io/en/latest/api/halotools.mock_observables.delta_sigma.html) function raises an unexpected exception when run in parallel. Thanks to @SandyYuan for pointing this out. Until this is fixed, users encountering this problem can run delta_sigma on a single core (by using the default argument value of `num_threads`=1), and/or use the [delta_sigma_from_precomputed_pairs](https://halotools.readthedocs.io/en/latest/api/halotools.mock_observables.delta_sigma_from_precomputed_pairs.html) function if workflow permits.