Count cic indexes with int64 to prevent overflow

[AlanPearl]

This PR will provide a solution to the ValueError in galtab issue 6 and does not degrade performance. I'll leave the example test script here, since it is too expensive to include as a unit test:

# NOTE Memory requirement: ~46 GB
import numpy as np
import halotools.mock_observables as htmo

sample = np.zeros((40_000, 3))
htmo.counts_in_cylinders(sample, sample, 1, 1, return_indexes=True, period=100)

[aphearin]

Thanks for the fix @AlanPearl!