Implement Inflow Boundary Condition

[ole]

Create boundary object that can apply a hydrograph to a boundary segment. The units are cubic meters per second and it should be time varying.

The challenge is to estimate the depth along the boundary segment based on Manning's formula and the slope into the domain.

Q = V*A
V = R^{2/3} * S^{1/2}/n

Where 
Q is the inflow [m^3/s]
A is the area of the boundary cross section
V is the estimated velocity
R is the perimeter width ~ estimated depth
S is the slope of the energy line ~ inward bedslope

the estimated quantities may be the result of an iterative process. Please comment on this everyone.