Implement Inflow Boundary Condition

[ole]

Create boundary object that can apply a hydrograph to a boundary segment. The units are m^{3/s 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.