Port enhancements to bitfield from stsci.tools

[mcara]

This PR enhances the treatment of very high bit flags in such a way that flags like 1<<63 (maximum supported by uint64 type) now work and flags larger than maximum supported by numpy.uint64 are ignored and again allow code to work without crashing. See #610 for more details.

[mwcraig]

Excellent, thanks for this @mcara! I'll take a look at it this Sunday.

[mcara]

@mwcraig Thanks! You may also want to take a look at my tests https://github.com/spacetelescope/stsci.tools/blob/master/lib/stsci/tools/tests/test_bitmask.py They are somewhat different from your tests for bitfield and provide 100% coverage of non-deprecated code stsci.tools.bitmask module. They also include these edge-case tests for "extreme" bit flags. However, you may not like how I designed them (i.e., parametrization, naming, etc.).

[mwcraig]

@mcara -- this looks good; I'm trying (and so far failing) to restart the tests....

I thought your tests looked great! Are you suggesting we add those here too? I'm ok either way.

Actually, broader question, which doesn't have to be answered here: would you be interested in migrating the bitfield stuff into astropy.nddata at some point?

[mwcraig]

closing/reopening to try to trigger a new build of the tests...

[mcara]

With regard to tests, I suggest several things and really it is up to you (it's your package) what you choose (in no particular preference from my side):

1. Replace your tests with my tests "as is".
2. Add my tests "as is".
3. You can cherrypick what what you want and make a PR yourself (or someone from your group) and rename/re-format my code.
4. I can make a separate PR for tests only an maybe you can fork it and modify it to you liking (naming conventions, notations, formatting, etc.) to do either 1. or 2. above.
5. I can add my tests to this PR to do either 1. & 2.

[mcara]

@mwcraig I hope I fixed the problem in the CHANGES.rst so that at least one failing test will pass. I do not know why the other tests are failing.

[bsipocz]

The current test failure is unrelated and is due to pytest being upgraded as part of installing mpl to a version that astropy 1.0.x doesn't support.