Include Stefan-Boltzman in net_radiation module

fix bug in readthedocs sphinx conf.py

Update to latest code from development version.

Include a 'Run PyTSEB' button to avoid some user confusion in the jup…

…yter notebooks

Update TSEB docstring

Fix bug in pyTSEB related to CalcSn_Campbell

Merge pull request hectornieto#7 from radosuav/TSEB

Code readability improvements in TSEB_2T and TSEB_PT models

Added API reference for readthedocs.com