Work on overland flow example
1 name change, 1 new method

New method set_fixed_value_boundaries_at_grid_edges().
Previous method set_closed_boundaries() renamed to
set_closed_boundaries_at_grid_edges() for consistency.
Trying to mend Voronoi & subgrids

Broke Voronoi yesterday; now repairing (still needs attention).
Modified grid.core_nodes property to check for existence of explicit
list self._core_nodes before searching BCs
Update manual index; get Voronoi working again

Modified method names in manual_index_alt_format.rst to reflect GT’s
last changes.
Restored a method in Voronoi.py to a @staticmethod to get code working
again. Added attribute self._num_core_cells for compatibility with
base.py.
Typo in RadialMG docstring

Replaced HexMG init in the RadialMG class docstring with Radial!
Changed to squeeze returned arrays.
Fixed unit tests for changes in raster_funcs.py.

Fixed
=====

* Updated unit tests for new functionality of:
  - node_id_of_cell_neighbor
  - node_id_of_cell_neighbor
Removed commented-out code.
Corrected function that calculates slope and aspect
Merge branch 'master' of https://github.com/landlab/landlab
Updata changes to radiation_field.py
Merge pull request #18 from mcflugen/fix-raster_funcs-docs

Fix raster funcs docs
Updated main landlab README file.
Merge pull request #20 from mcflugen/fix-readme

Updated main landlab README file.
Changed to get long_description from README.md.
Merge pull request #21 from mcflugen/fix-readme

Changed to get long_description from README.md.
Pointing to the updated radiation file

Updating the document file to point to the appropriate radiation code
(/components/radiation/radiation_field.py)
Add to docs, mend get_active_link_connecting...

Mended get_active_link_connecting_node_pair.

Modified docstring for create_node_array_zeros so it’s clearer how to
get your data onto the grid fields.

Added placeholder methods for .at_node & [‘node’][…] to the manual
index.

Created a short document for the readthedocs on the woes of Python on
Linux, after some negative experiences this morning.
Cleaned up ModelGrid diffusion examples

Also created _setup_cell_areas and _dx in HexModelGrid, and added
boundary_nodes property to base.
Merge branch 'master' of https://github.com/landlab/landlab

Conflicts:
	landlab/grid/raster.py
Updating file