redo flow routing GRASS assignment

[petrasovaa]

• re-do the assignment to highlight the difference between filling and not filling the sinks,
• re-organize flow assignment to make it clear that filling is not needed and comapre accuracy with filling and LCP
• use larger threshold to eliminate smaller streams - or keep them there so that students have something to write about
• stream extraction by running r.watershed over cost surface with bridges with low cost
• flow assignments - indicate which is D8 and which is d-inf (or pose it as a question)
• check d.vect streams col=green for visibility
• add d.rast.num to see the values for flow accumulation to select the outlet