Stats-arrays

* Merged [PR 13](https://github.com/brightway-lca/stats_arrays/pull/13). Move argsort to save on execution time. Thanks Loisel!
* Merged [PR 12](https://github.com/brightway-lca/stats_arrays/pull/12). Update links in readme. Thanks mfastudillo!
* Merged [PR 10](https://github.com/brightway-lca/stats_arrays/pull/10). Fix flaky test on distributions::extreme.py, distributions::gama.py, and distributions::student.py. Thanks lonly7star!

* Updates for changes in Numpy API

* Merged [PR 5](https://bitbucket.org/cmutel/stats_arrays/pull-requests/5/use-meanround-0-instead-of-round-mean/diff), fix rounding function.

* Improve speed of lognormal CDF and PPF under common conditions

* Restore Python 2.7 compatibility

* Several bug fixes from Daniel de Koning
* Start improvement of beta distribution