Test with branching

Trying my hand on branching
Revert "Test with branching"

This reverts commit 9c14122a7687cd398f8d0ba7960bd0b7c139e08a.
Rectified Priestly Taylor method for calculating PET

corrected Priestly Taylor method for calculating potential
evapotranspiration. This is the first edit. Future commits will try to
include documentation.
Minor changes

minor updates to radiation and soil moisture components
Intermediate commit

Committing some changes to radiation, PET and very minor changes to Soil
Moisture
Intermediate commits

radiation_field.py: updating slope and aspect calculations
PET: adding few more fields
SM: very minor update of one default parameter
Copy of Master branch code

Copying the code from master branch to implement N-S mirror corrections
Merge remote-tracking branch 'origin/Sai_PET_09Jul14'

Conflicts:
	landlab/io/esri_ascii.py.

Sai: I have manually deleted what was shown under my branch's version of
the conflict. Hopefully, I have not messed up the code.