add deposit porosity feature

[amoodie]

When trying to broadly model real world scenarios, it would be helpful to include the deposit volumetric porosity as a component of the model. Presently, there is no porosity.

• add porosity = 1 as a new default model parameter
• pass porosity to sediment routers
• account for porosity in parcel->deposit and deposit->parcel mass transfers
• verify that consistency checks are not broken when porosity=1
• add tests that have bed elevation change that reflects the sediment concentration adjusted by a porosity value