feat: add VALID_LOCATIONS to ModelGrid
feat: add VALID_LOCATIONS to ModelGrid

Note that this value should be overridden on grids that don't support
the default locations.

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