Fix for issue 203: Make ud_grade work on python3 for Nside>128. 

This trivial change forces integer division where needed.

No test is provided since I do not know what a good one would be.  In this case ud_grade throws an error.  A test could be written to verify that ud_grade runs for Nside>128 but this is a very specific test for a very specific problem.  It wouldn't even test that ud_grade works correctly, just that it works at all.  This doesn't seem a good candidate for a general test.