#516 - unit tests

I was looking for something to do while traveling yesterday, and I decided to take on this long-neglected issue on unit tests of the PowerSpectrumEstimator class.  While I was offline and working on this, I used the coverage tool to find some other places where unit test coverage was incomplete, and filled those in too (but I did this entirely haphazardly, not systematically, and we still have some bits of code that lack unit test).

No rush on this one, I think, but I wanted to make the PR while it was all fresh in my mind.
assert_allclose doesn't work with older numpy versions that I think we are still trying to support.  Can these be changed to almost_equal?  Or if not, then guard with a try/except block like we do in test_lensing.py.

It looks like there is one in test_bandpass.py as well that escaped notice.  If you don't mind, could you change that one too?