Added var_help method; Added class properties.
Added var_help method; Added class properties.

* New var_help method prints a summary of a variable.
* Changed many methods (input_var_names, output_var_names, etc.) to be class
  properties.

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