* Added new unit tests.
* Added new unit tests.

Added
=====

* Unit tests for get_diagonal_list and get_neighbor_list.
* Unit tests for ModelGrid.zeros



Carralex/landlab,cmshobe/landlab,Carralex/landlab,landlab/landlab,ManuSchmi88/landlab,landlab/landlab,RondaStrauch/landlab,amandersillinois/landlab,landlab/landlab,RondaStrauch/landlab,csherwood-usgs/landlab,amandersillinois/landlab,RondaStrauch/landlab,cmshobe/landlab,Carralex/landlab,laijingtao/landlab,csherwood-usgs/landlab,ManuSchmi88/landlab,SiccarPoint/landlab,cmshobe/landlab,ManuSchmi88/landlab,SiccarPoint/landlab,laijingtao/landlab,decvalts/landlab,decvalts/landlab