REL tag version 3.2

Release candidate 1 of version 3.2

v3.1 Final

Bump version to 3.1rc4

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