DOC Bump version number to 3.1rc3. Update release notes. Add missing …

…NBs to example docs.

Version 3.1 release candidate 2

Bump version.

Version 3.0 release

Merging (pymc-devs#1635)

Fifth release candidate of PyMC3 3.0

Release candidate 4 of PyMC3 3.0

Tweaked the BEST notebook (pymc-devs#1568)

MAINT Change 3.0 to 3.0.RC3

Release candidate 2 of PyMC3 3.0