new driver for sed_flux_dep

New driver for sed_flux_dep_incision, allowing perturbation of an
existing grid without making a new one.
Stability testing in stream_power, etc

Main thing: stream_power now performs some stability testing, to
prevent upstream nodes from incising below any node downstream of it.
This has involved changing the input arguments… sorry!

Also some minor tweaks to the sed_flux_dep drivers to allow saving of
long profiles.
Merge branch 'sed-flux-dep'