#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.
Good point.  I just converted to ```assert_array_almost_equal```.  I had avoided it because the docstring for that method kind of says that ```assert_allclose``` is preferred.  But for the sake of compatibility you're right that it makes sense to use ```assert_array_almost_equal```.  (I took care of test_bandpass.py also.)