Expose allow_huge in chi2_shift function and add test function

[adrtsc]

This PR implements the changes to expose allow_huge in the chi2_shift function as requested in issue #60. When allow_huge is enabled, the function can now run on much larger arrays. In short, I made following changes:

1. Add "allow_huge" argument to image_registration/chi2_shifts.chi2_shift with a default value of False and updated docstring.
2. Add "allow_huge" argument to image_registration/chi2_shifts.chi2n_map and updated docstring.
3. Add "allow_huge" argument to any function calls to image_registration/chi2_shifts.chi2n_map
4. Add "allow_huge" argument to any function calls to image_registration/fft_tools/correlate2d
5. Add "test_issue_60" function to image_registration/tests/registration_testing.py

Happy to make more changes if necessary.