SimplyQ: in the reach flow equations, replace EffectiveReachLength with the real reach length?

[LeahJB]

In SimplyP v0.3 we replaced reach length with effective reach length in the reach flow equations. The intention was that the effective reach length should represent the average distance travelled by water within the reach (i.e. the full reach length for water from upstream, and about half the reach length for water entering from the sub-catchment). However, it makes the reach flow equations more stiff, and implicitly implies that the reach water depth is twice what it really is (for headwater catchments).

We need to think about reach water travel times a bit more generally, including this. Probably we should go back to just using the real reach length, and just make sure the reach lengths aren't too big (see Issue #20 ).

[LeahJB]

Also factor out reach water transit time into its own equation.