Pure elevation based simulation

[changliao1025]

When pyflowline doesn't have the river network information, the hexwatershed simulation fells as the pure elevation based.

[changliao1025]

by default, if pyflowline has no river ino, it becomes a mesh generator.
but the pyhexwatershed will still produces one watershed using dem.
so there should be one pybasin instance created somewhere.