Updates to Model Overview and References

* Clarified descriptions for partial veg cover fraction and time-varying veg parameters.
* Added references to Bohn and Vivoni (2015).
* Added schematic of partial veg cover fraction.
* Numbered all figures in the model overview.
* Clarified descriptions of a few other features.
* Added references to Francini and Pacciani (1991) for soil evaporation and ARNO baseflow; added reference to Brooks and Corey (1964) for gravity-driven drainage.
* Added links from all references to the References.md page where links were not present.
* Added references to Bohn and Vivoni (2015), Francini and Pacciani (1991), and Brooks and Corey (1964) to the References.md page.
* Alphabetized references on the References.md page.

Author: tbohn