* Python 3 compatibility.
* Allow cross-polarization spectra to be calculated as long as they
aren't beam-normalized; polarizations now specified as polpairs.
* Time-dependent noise power spectra capability in generate_noise.
* Methods to fetch redundant baselines/blpairs from UVPSpec objects.
* Exact normalization mode (Saurabh Singh) with optimization (Ronan
Legin).
* Updated covariance handling, incl. averaging and analytic variance
(Jianrong Tan).
* New 'lazy covariance' weighting mode (Aaron Ewall-Wice).
* Fix bug where little_h was passed incorrectly (Zachary Martinot).
* Store additional statistical info in stats_array (Duncan Rocha).
* Add delay wedge plotting function (Paul Chichura).
* Various minor interface changes and improved tests.