Add cythonized udunits2

[mcflugen]

This pull request cythonizes the udunits2 library as pymt.udunits2. This will, hopefully, provide a more robust unit conversion module that works on multiple platforms and is under our control (we were having issues with cfunits). Thus far, our udunits2 module may not have all of the functionality of cfunits, but it's enough for our purpose at the moment.

Still to do:

• Ensure all the tests are passing and the new module is well tested
• Add documentation for the new module
• Deploy a binary distribution to PyPI as part of our CI
• Remove the old pymt.cfunits package