fixing unit tests, doctests

Correcting unit tests to reflect masked array returns.

Also, route_flow_dn can no longer have the grid cell size overridden
(as this makes little sense in any application I can think of, and is
quite confusing to me…!) Note this can be simulated using the runoff
rate anyway.
Suppress weaves

1. Pending resolution of typecasting issues raised in Landlab Issue
#81, all scipy weaves now revert to pure Python. Weaves will be
returned once stable.

2. Corrected type of failure from several try statements in
stream_power.py.
Merge branch 'sed-flux-dep'

Conflicts:
	landlab/grid/raster.py
	landlab/grid/raster_funcs.py