Added description of the second order boundary condition, reduced two layer model and the in-canopy model

[menno94]

• Description of the second order boundary conditions (Appendix/numerical implementation/boundary conditions/Description second order waves).
• Description of the reduced two layer model (Processes and model formulation/Reduced two layer model (nh+)).
• Description of the formulation of the in-canopy model (Processes and model formulation/Shallow water equations/Porous in-canopy flow)
• Description of the input of the in-canopy model (Input description/Porous in-canopy model input)

[keesnederhoff]

Approved Menno his contribution to the XBeach manual.