added mappers from links to nodes.
added mappers from links to nodes.

added mappers that handle the following cases:

inlink sums to nodes
inlink average to nodes
inlink min to nodes
inlink max to nodes
outlink sums to nodes
outlink average to nodes
outlink min to nodes
outlink max to nodes
average of all links to nodes.

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