Sed flux dep, core functionality & testers

The sed_flux_dep_incision component now appears to be in acceptable
working order. Note there is no internal timestep checking; if you’re
seeing numerical instabilities, turn the timestep down!

Note incision is permitted in the transport limited condition. Should
it be?

Also found a bug in stream power when handling arrays not fields.
Simple fix, but might be in other modules too.
Merge branch 'sed-flux-dep'