repair broken fn calls

Repaired, (then commented out, as were never used!) calls to a now
deprecated and removed grid method in dem_support.
Explicit stream power

components/stream_power/stream_power.py is now a functional, explicit,
forward stream power component. The component is designed to take
single flow directions and local slopes and perform stream-power type
pure detachment limited channel incision.

This component will form the base of a sediment flux dependent
detachment limited module.