fix r.drain and add a submodule for r.drain that does not need input point vectors
add a r.cost submodule that uses a raster input as starting point(s)
Merge pull request #1472 from gioman/r_cost_raster

[processing] add a r.cost submodule that uses a raster input as starting point(s) + fix r.drain and add a submodule for r.drain that does not need input point vectors