Added function documentation.
Added function documentation.

Added
=====

* numpydoc-style documentation for functions in raster_funcs.py.
* Added doctests for functions.

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