Work associated with adding support for Python 3 to meas_extensions_ngmix.
Port afw to Python 3
Port Shapelet to Python 3
Port meas_base to Python 3
Port meas_algorithms to python 3
Check ngmix works on Python 3
Support Python 3 migration
Ported package and passed all python 2 and 3 tests in Jenkins.
Minor comments on PR. Mainly docstrings.