Python 3 update

Python 3 should now be supported by all of pyFFTW. If you run into problems, please let me know!

There are also updates from the past long while are now included, such as fixes to coherence modeling, some pulse types, DFG without pyFFTW. Also there are a few new examples and updates to old ones.

This should technically be a release-candidate, but as I believe that it is an improvement to what is currently on PyPi, I am tagging it as a release. Please let me know if there are any problems!

Incremental update #1

Improvements from development repository are merged, including significant improvements to three-wave mixing. More flexibility is also added for specifying waveguide properties for four-wave mixing. Miscellaneous improvements to naming conventions and helper functions.
Upgrading from v0.1 to this release requires some code updates to code, but only superficial changes will be needed.