Fixed small mistake setting up PCSE in quickstart guide.
- Added util.penman_monteith to calculate the PM reference ET0
- Added util.reference_ET as a wrapper around util.penman and util.penman_monteith, combining the ref ET calculations for E0, ES0 and ET0.
- Added unit tests for penman monteith based on CGMS reference results.
- Updated the CABOWeatherDataProvider and NASAPowerWeatherDataProvider to use the new Penman-Monteith ref ET.
Fixed references to python UserDict to support both python2 and python3
File case name change to be consistent with the test code
Fixed problems with UserDict when using it in python 2 and python 3. Code should be compatible with both now.
Merge pull request #1 from marekpetrik/python3_compatibility

Python3 compatibility
Merge pull request #1 from ajwdewit/develop

Include penman-monteith ET