doc strings for mappers.py
doc strings for mappers.py

Added more descriptive doc strings for the mappers.py file and
functionality.

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