allow reaccumulation of flow; more tests
allow reaccumulation of flow; more tests

Add reaccumulation option so that it is possible to update all the flow
fields (e.g. ‘drainage_area’) from within the LakeMapper.

Add a few more doctests. Everything now passing on py2 & py3.

cmshobe/landlab,amandersillinois/landlab,cmshobe/landlab,landlab/landlab,landlab/landlab,landlab/landlab,amandersillinois/landlab,cmshobe/landlab