ENH: Adding new permeable boundary conditions
- prghPermeableAlphaTotalPressure for p_rgh - pressurePermeableAlphaInletOutletVelocity for U - new helper class for pressure-related BCs: updateableSnGrad
- prghPermeableAlphaTotalPressure for p_rgh - pressurePermeableAlphaInletOutletVelocity for U - new helper class for pressure-related BCs: updateableSnGrad