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
Working on link re-orientation option
Added voronoi.py method reorient_links_upper_right() to create grid with all links pointing up and/or right
Created oriented_hex_lca.py
Cleaned up oriented_hex_lca.py, and added oriented-hex example grains.py
Intermediate commits

radiation_field.py: updating slope and aspect calculations
PET: adding few more fields
SM: very minor update of one default parameter
Changed so that returned array is flipped.
Updated unit tests for read_esri_ascii change.
Merge pull request #60 from landlab/fix-esri-ascii

Fix read_esri_ascii
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.
added Burroughs slope and aspect

added Burrough's slope and aspect algorithms to raster.py
There are issues with Horn and Best fit plane methods to calculate slope
and aspects. slope_aspect_routines_comparison.py will help understand
the issues. elevation_NS.npy is an elevation profile required for this
code.
updated slope and aspect calculation routines

updated to use Burrough's slope and aspect calculation routine. This
code now uses numpy arrays getting rid of traditionally used for loops.
cleaning unnecessary files

getting rid unnecessary files
Implemented continuous-time stochastic lattice-gas model prototype; also improvements in hex plotting
Completed and tested cts_lattice_gas.py