Include gravity impact on the pressure at the boundary

What this MR does / why does DuMux need it:

If gravity needs to be included, assigning a fixed pressure at the outlet of the free-flow channel could lead to non-symmetrical velocity profile near the outlet and even back-flow. This problem is more obvious when the inlet velocity is low.

To fix it, we need to include the impact of the gravity as hydrostatic pressure and add it to the outlet pressure specified by user.

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